Edit: Replaced "command mode" with "normal mode" - @Jim Steward: thanks for pointing out that incorrectness
Short answer: If you are in normal mode and want to insert text after a character press a
. Press A
to directly jump to the end of the line and to insert characters after the last character. In vi, afaik, it is not possible to place the cursor on a column after the last character in the current line (if you are in normal mode). In vim however there is an option named virtualedit, which can enable you to do so (see :help virtualedit).