While 行きます means just go somewhere, 行ってきます means the same but also implies to come back. The reason why it means not only go but also come back is that 行ってくる consists of 行く(go)+くる (戻って来る come back.) However, it doesn't mean promising to come back. Even though the term of this expression is implying to come back as well, a lot of people use this as just go.
If I say I go to the library, I am just explaining to go there,
on the other hand, if I say "libraryに行ってきます," This also means 行く+来る(戻ってくる)go to the library and will come back to here.
You can use 行きます under any circumstances.
私は今週の土曜日、仕事に行く/行きます = I go to work this Saturday.
彼は今年の夏、アメリカに行きます = He goes to America this summer.
彼女は昨年、北海道へ行きました = She went to Hokkaido last year.
March 13, 2015 at 7:45pm