This answer summarises the other answers and comments of this question, and adds extra information based on the Vim documentation and the Vim wiki. For conciseness, this answer doesn't distinguish between Vi and Vim-specific commands.
In the commands below, "re-indent" means "indent lines according to your indentation settings." shiftwidth
is the primary variable that controls indentation.
>> Indent line by shiftwidth spaces
<< De-indent line by shiftwidth spaces
5>> Indent 5 lines
5== Re-indent 5 lines