OK, so I know very little about Islam, but this question doesn't seem to be Google-able. I've seen some people in the Qur'an described with a.s. after their name - such as Nuh a.s. What does that mean?
"a.s." is short for "'Alayhi al-Salaam" (عليه السلام), which is an Arabic phrase meaning "Peace be upon him". It is often used by Muslims after the names of the prophets as a term of respect.
As @goldPseudo correctly mentioned, "a.s." is the short form of "'Alayhi al-Salaam" (عليه السلام), which is an Arabic phrase and it means "Peace be upon him" and actually it is often used by Muslims after the names of the prophets as a term of respect.
On the other hand, as well as the front of prophets' names, it is often used by Shia Muslims for Shia Imams, too. For instance:
Imam Ali (a.s.) said ... So, it also means "peace be upon him" for Shia Imams.
A.S. means "Peace be Upon Him" used after the mention of Prophets and Angels (like Jibreel –Gabriel- A.S). As mentioned, Shia uses the term on their Imams. The term can be used on the Beloved Prophet Muhammad Salla llahu alay-hi wa sallam but (salla llahu alayh-hi wa sallam is recommended). See Wikipedia for more on it.