1. What is a denomination—specifically a Christian denomination?
2. Which denominations are included in Christianity?
Here's the definition of "denomination" from American Heritage Dictionary in Dictionary.com.
"A large group of religious congregations united under a common faith and name and organized under a single administrative and legal hierarchy."
Check this site for a lengthy list of Christian denominations.
http://en.wikipedia.org/wiki/List_of_Christian_denominations
This site compares the major denominations in the U.S.
http://www.history.com/encyclopedia.do?articleId=227134