paste

paste takes two or more files as input, concatenates each sequential line from the input files, and outputs the resulting lines. It can be useful to create tables or columns of text.

join

join is similar to paste, but it uses a field (the first, by default) in each input line to match up what should be combined on a single line.

tee

tee prints its input both to a file and to the screen. This is useful when you want to create a log of something, but you also want to see it on the screen.