There are many timelines in the Gundam universe.
Universal Century is the English name for the original timeline of the Gundam anime metaseries. It is also called "Uchuu Seiki", meaning "The Era of Space", in Japanese, but the official English name Universal Century is always used to tag along the Japanese kanji name and is often abbreviated as "UC".
The Universal Century universe remains by far the most developed one, with four TV series, four OVA series, fifteen theatrical releases, and various specials.
There are two other timelines that - when a certain point in time is reached (UC 155) - slightly diverge from the original one:
After Colony is a timeline corresponding to the construction of the 195 space colonies in Mobile Suit Gundam Wing. It is often abbreviated as "AC." The After Colony timeline takes after the Universal Century in that an Earth Federation-like government (Earth Sphere Alliance and later the Organization of the Zodiac), retains control of the space colonies and is more oppressive. The colonies struggled for freedom, and the Gundams were built as way of defending the plight of the colonies to escape this oppression.
The After Colony universe is the third most developed, with one TV series, one OVA series and a compilation movie.
After War is an timeline noted for its distinct apocalyptic world. However, it has many similarities with the Universal Century. After War is the only timeline, besides the U.C., to feature Newtypes as an important element.
The Correct Century is known as Seireki in Japanese. Seireki is a wordplay on the Japanese term for the Common Era (C.E.) Western calendar system (西暦; also pronounced Seireki). Its kanji literally means "Correct Calendar" and is unofficially interpreted as "Correct Century" by English-speaking fans.
Cosmic Era is an alternate timeline. It is often abbreviated as "C.E."
The Cosmic Era universe is the only Gundam universe after the original Universal Century with multiple TV series. It is also the second most developed, with two TV series, one ONA and two compilation movies.
Anno Domini is an alternate timeline for Gundam 00 which uses the Gregorian calendar.
The Advanced Generation (often abbreviated as "AG" or "A.G.") is the alternate timeline for Mobile Suit Gundam AGE. This timeline describes to be several hundred years since mankind had migrated into space and living inside space colonies. Wars on earth had long ended, and the world seems to be in a peaceful era.