Sindarin is the Grey Elven Tongue of Middle-earth, and the most common Elvish language used by the Third Age. It was the language of the Eldar that lingered on the shores of Arda and didn't pass over the sea in the beginning. Sindarin is closely related to Quenya. Both began as the Primitive Elvish language, but because Sindarin was used by the Grey Elves (Sindar) in Middle-earth, it became long estranged from Quenya, and in later times when both languages were again used in Middle-earth ages after, the two had become extremely different. But there were still many similarities.
If you don't already learn do Sindarin/ Quenya, why not start now? They're both absolutely wonderful, as Becky'll tell you too, and after doing Elvish for more than a year, it's becoming an obsession... :)