Who is Lord Sri Caitanya?

Lord Sri Caitanya (also called Lord Gauranga due to His golden complexion) is a combined form of Sri Radha and Sri Krishna, stating more correctly, He is Sri Krishna in the mood of Srimati Radha Rani. He comes down from Swetadvipa the highest loka in Krishna Loka (mentioned in the Vedas) and is situated beyond Vaikuntha. Svayam Bhagavan Sri krishna comes down only once in a day of Lord Brahma (4.32 Billion earth years) in a specific Dwapar Yuga (last one happened about 5,000 years ago) in a specific Manvattara, and in the Kali Yuga that follows it, Lod Sri Caitanya comes down. When Sri Krishna comes down, all of His other forms such as Sriman Narayana (Vasudeva form), Maha Vishnu, Vishnu and every one of His other expansions also comes down and attract respective bhaktas in those moods as well. For example, to those who had svakiya bhava (marital relation) never saw or participated in the leelas of Gopis who had parakiya bhava.

Why Lord Caitanya is so special?

He is so special because He is only one who can give Vraja Prema. It is stated in Caitanya Caritamrta repeatedly, that no other incarnation, even Sriman Narayana who is a Vaudeva expansion of Sri Krishna cannot give this Prema. The reason is that Sri Radha Rani does not come in any other form of Lord Krishna. Also another special aspect of Lord Sri Caitanya is that while Krishna brings mostly Gopis, Lord Caitanya brings mostly the Manjaris whose bhavas are praised as higher than Gopi Bhava. 

Rarely enough His form and nature is described in the Vishnu Sahasranama also but hardly anyone in any sampradaya knows the real meaning of that sloka:

suvarna-varno hemango

varangas candanangadi

sannyasa-krc chamah santo

nistha-santi-parayanah

"The Lord (in the incarnation of Gaurasundara) has a golden complexion. Indeed, His entire body, which is very nicely constituted, is like molten gold. Sandalwood pulp is smeared all over His body. He will take the fourth order of spiritual life (sannyasa) and will be very self-controlled. He will be distinguished from mayavadi sannyasis in that He will be fixed in devotional service and will spread thesankirtana movement.” (Mahabharata, Visnu-sahasra-nama-stotra)."

Lord Caitanya Mahaprabhu appeared about 500 years ago in the land of Nawadwip Dham in West Bengal as forecasted in many satras including the one above. So we are extremely fortunate in this life to receive the mercy of Lord Sri Caitanya by the causeless mercy of our Guru parampara running down from Lord Caitanya!

Lord Sri Caitanya is the Yuga Avatar of this Kali Yuga

He is the one who officially inaugurated the Nama Sankirtan movement which is the Yuga Dharma of this Kali Yuga

He came to taste the mellow of Prema Bhakti especially the Maha Bhava which only Srimati Radha Rani possess.

He came down in the form of His own devotee (of Krishna) to teach us abour pure devotion to Krishna.

According to Sri Caitanya Mahaprabhu, except Srimad Bhagavatam which came from Veda Vyasa in his most mature stage (at the end of his life by the special mercy of Narada Muni), Srimad Bhagavatam is the only purana that is currently pure and thus we ever need. All other puranas are not pure due to mix of three modes of material nature and hence should not be followed. He disapproved the false propaganda of Vishnu Purana that Krishna is an avatar. In contrast, Lord Caitanya declared that Sri Krishna who Himself is the avatari where every expansion including Lord Narayana (the Vasudeva expansion) appeared.