Who gives titles to whom?
The Guru gives the title to His student in most of the cases[a]. Sometimes the saint Himself gives Himself a name. Sometimes the disciples give the name. There is no hard and fast rule for this. For example, Sri Ramana Maharishi did not receive His name from any Guru, but rather He was affectionately called so by His devotees on account of the Transecendental Experiences he narrated and the Silence with which He transformed all.
Which one is the highest among them?
Going by pure translation and by definition the highest is Satguru[b]. The Satguru is automatically a Paramhansa, a saint and a Avatar. he is the highest of the highest. He is the indwelling Antaryamin and the Great Parameshwara Himself.
But that does not mean anything. Because the titles are usually given by others. Thus in most cases titles are just used as an honorary to the saint, and there is no need to demarcate one saint as greater than the other!
What is the significance of the title?
To the devotee of the saint, the title is his expression of love for the saint. When we say 'My dear Sri Sri Sri Maharishi Guru maharaj' it sends a thrill down our spine with the thought of the Guru.
To the non-devotee, the title has no other significance than a name, a way of referring to the saint, in honor of His holiness.
What is the difference among them (briefly)?
Just going by pure definition and translation, here are some titles and some saints who are called by this title (in no way that means one saint is inferior or superior to the other, it just example of one who is called by that title, that's it all the best)
- Acharya - Teacher (of any field, not necessarily spirituality). Example: Dhronacharya, the teacher of Arjuna.
- Shankaracharya - A monk of the ashrams established by Sri Adi Shankarcharya. Example: Chandrasekharendra Saraswati.
- Godman - A God-realized saint, or an avatar, i.e. A man who is believed to be God Himself. (It is usually used as a negative term thus I have no examples).
- Guru - A spiritual teacher or spiritual master. Example: Sri Yukteshwar Giri Swami was the Guru of Swami Paramhansa Yogananda.
- Jagad Guru - A spiritual teacher for the whole world. That is He distributes His teaching freely to all, nobody needs to receive initiation for receiving His teachings! Example: Sri Krishna whose bhagavad gita is available to all.
- Maharishi - A great sage. Example: Maharishi Vyasa, whose greatness requires no description either.
- Pandit - A learned man.
- Paramhansa - A God-realized Supreme Saint. He has transcended the Three Gunas. Example: Sri Ramakrishna Paramhansa.
- Paramguru - The Guru of Gurus. Example: Adi Shankaracharya is the Param Guru for all the shankaracharyas.
- Rajarishi - A great sage who is a King as well as a sage. Example: Vishwamitra.
- Rishi - A great sage. Example: Agastya.
- Sadhaka - A spiritual aspirant. Example: you and me.
- Saint - A holy man. Example: So many saints are there.
- Sannyasa - A renunciate or monk. Example: Shankaracharya.
- Satguru - The God Himself come as a human, Ishvara in human form is Satguru. Example: Sri Krishna.
- Swami - A monk of the order of Swami's established by Shankaracharya. (Although I believe other also uses it)
- Yogi - A human who practices yoga and attains realization through Yogic principles. Example: Kriya Yogis.
[a]: Autobiography of a Yogi, Paramhansa Yogananda, Chapter 1, footnote 3.
[b]: Satguru is God manifested in man, according to 'Rebirth is Optional' by Om Khera.