Jainism believes that universe and all its substances or entities are eternal. It has no beginning or end with respect to time. Universe runs own its own accord by its own cosmic laws. All the substances change or modify their forms continuously. Nothing can be destroyed or created in the universe. There is no need of some one to create or manage the affairs of the universe. Hence Jainism does not believe in God as a creator, survivor, and destroyer of the universe. However Jainism does believe in God, not as a creator, but as a perfect being. When a person destroys all his karmas, he becomes a liberated soul. He lives in a perfect blissful state forever in Moksha. The liberated soul possesses infinite knowledge, infinite vision, infinite power, and infinite bliss. This living being is considered God of Jain religion. Every living being has a potential to become God. Hence Jains do not have one God, but Jain Gods are innumerable and their number is continuously increasing as more living beings attain liberation. Jains believe that from eternity every living being (soul) is attached with karma. The soul suffers due to existing karma. While suffering it acquires additional karma because of its ignorance of its true nature. The main purpose of the religion is to remove the karma through knowledge of self and become liberated soul. There are many types of karma. However they are broadly classified into the following eight categories: 

Mohniya karma-It generates delusion in the soul in regard to its own true nature, and makes it identify itself with other external substances.
Jnana varaniya karma-It covers the soul's power of perfect knowledge.
Darasna varaniya karma-It covers the soul's power of perfect visions.
Antaraya karma-It obstructs the natural quality or energy of the soul such as charity and will power. This prevents the soul from attaining liberation. It also prevents a living being from doing something good and enjoyable.
Vedniya karma-It obscures the blissful nature of the soul, and thereby produces pleasure and pain.
Nama karma-It obscures the non corporeal existence of the soul, and produces the body with its limitations, qualities, faculties, etc. 
Gotra karma- It obscures the soul's characteristics of equanimity, and determines the family, social standing, and personality.
Ayu karma-It determines the span of life in one birth, thus obscuring soul's nature of eternal existence. 

While travelling on the path of spiritual progress, a person destroys all eight types of his karmas in the following sequence:

First it destroys Mohaniya (delusion) karma then Jnana varaniya (knowledge), Darasna varaniya (vision), and Antaraya (natural qualities) all three together At this time, he attains Keval_jnan and he is known as Arihant, which is also known as Tirthankara, Jina, Arhat, Kevali, or Nirgantha. Lastly the remaining four karmas namely Vedniya (pleasure and pain of the body), Nama (body), Gotra (social standing), and Ayu (life span) are destroyed. At this time, he attains total liberation and he is known as Siddha. 

The first four karmas are called Ghati karmas because they obscure the natural qualities of the soul. The last four karmas are known as Aghati karmas because they do not affect the qualities of the soul, but they are associated to the physical body of the soul. Once a person destroys all his Ghati karmas, he will definitely destroy all of his Aghati karmas before his death. No fall back can occur after the destruction of Ghati karmas.

Both Arihant (Living Human being) and Siddha (Free Soul) are considered Gods in Jainism. Arihant When a person destroys his four Ghati karmas, he attains keval jnana. He has regained the original attributes of his soul, which are perfect knowledge, vision, power, and bliss. He is omniscient of the past, present and future forms of all entities (living and nonliving beings) of the universe. He is still a human being. He remains in the state of blissful condition for the rest of his life. Arihants are classified into two categories: 

Tirthankar
Simple kevalin

Tirthankar
Immediately after attaining keval jnana, if a person establishes the four fold religious order of monks, nuns, sravaks (male householder), and sravikas (female householder) is known as Tirthankar. He preaches the Jain philosophy, religion, ethics, conducts to his followers.

Jainism believe that Twenty four Tirthankars are born during each descending and ascending part of the time cycle in this region (known as Bharat Kshetra) of the universe. No two Tirthankaras exist at the same time. Generally a Tirthankara is born when the religion is at its depression state. The new Tirthankar revives the same Jain philosophy. He gives a different form to the religion practice, which depends upon the time, place, and the social behavior of the human society of that time.

Example:

Lord Mahavir preached five great vows for ascetics, whileLord Parshva preached four great vows. The vow of celibacy was included in the non possession category by Lord Parshav.
Simple_kevali The only difference between Tirthankara and simple kevali is that the latter does not establish the religious order. He remains in the state of perfect blissful condition for the rest of his life after attaining keval_jnan.

In the religious scriptures, the name Arihantas and Tirthankaras areinterchangeably used because simple_kevalis do not play any active roles in the religious order. Tirthankar is also known as Jina, Arihant, Arhat, Arhant, or Nirgrantha.

Tirthankara: Founder of four-fold order of monks, nuns, laymen, and laywomen.
Jina: One who has conquered his, inner passions such as desire and hatred.
Arihant: One who has destroyed his inner enemies such as greed, anger, desire, and hatred.
Arhat: One to whom nothing can be secret.
Arhant: A spiritual acquirement leading a man to the state of an Arihant.
Nirgrantha: A religion of One who has gotten rid of all knots or attachments. Both Tirthankars and Simple_kevalis (all Arihants) become Siddha after nirvan (death). All Siddhas are equal in qualities. Siddha Both the Tirthankaras and simple kevalis (all Arihants) destroy the remaining four Aghati karmas at the end of their present life. After their nirvan (death) all of them are known as Siddhas. They are totally free and liberated. They are free from the birth and death cycle. They do not possess body. They do not feel pleasure and pain, or joy and sorrow. They live as a pure soul in an ever-lasting blissful condition at the top of the universe (Lokakas) known as Moksha.
The quality and attributes of all Siddhas are same. However, they still maintain their unique identity and form.
Example:Lord Mahavir's soul as a siddha possesses the same qualities as of Shri Gautam Swami or Shri Bahubali's soul. However their souls remain unique individual and have different forms. 

Jain Gods
Both Arihants and Siddhas are considered Gods of Jain religion. Arihats are perfect human beings and preach the Jain religion to the people after attaining keval_jnan. After death they become Siddhas. All Siddhas are perfected souls, living forever in a blissful state in Moksha.

Question
In the Namokar Mantra we pray to the Arihants (Tirthankara) first and the Siddhas second. Even though the Siddhas are perfected souls, have destroyed all (both Ghati and Aghati) Karmas, and are at the highest spiritual stage. While the Arihantas have destroyed only four Ghati Karmas and are at a lower (thirteen gunasthan) spiritual stage.

Answer
Arihants (Tirthnkaras) after attaining keval jnana (after destroying four ghati karmas), establish the four fold order of Jain religion. They preach the Jain philosophy, ethics and conduct. They explain the path of liberation and the qualities of the perfected soul or Siddhas. Without the teachings of Arihantas we would not have known Siddhas or liberation. For this reason we pray Arihantas first and Siddhas second.

The Four Fold order 
Monks and Nuns (ascetics):They practice self control and have given up all desires and earthly possessions become the spiritual practicer and teachers. They follow strictly five great vows (maha vrats).

Sravaks and Sravikas (lay followers):They are not required to renounce the world, but are expected to discharge household duties by honest means and live a progressive pure life. They follow the twelve vows of lay people.