Emacs normally uses both tabs and spaces to indent lines. If you prefer,
all indentation can be made from spaces only. To request this, set
nil. This is a per-buffer variable;
altering the variable affects only the current buffer, but there is a
default value which you can change as well. See Locals.
There are also commands to convert tabs to spaces or vice versa, always preserving the columns of all non-blank text. M-x tabify scans the region for sequences of spaces, and converts sequences of at least three spaces to tabs if that is possible without changing indentation. M-x untabify changes all tabs in the region to corresponding numbers of spaces.