Commands take one of two forms:
The second case is case-sensitive; \Gamma is different from \gamma. These commands are terminated by a nonletter, i.e., a blank, a number, a punctuation mark or special character.
For commands which produce text a trailing blank is interpreted as the end of the command, which does not automatically create a space, so you may have to do something to create an interword space after this text.
Commands may have mandatory arguments, which are input in braces ({}). They may also have optional arguments, which are input in square brackets ([]).
See:
These must be used for entities which are not already defined. To redefine something use, for example, \renewcommand.