If you don't specify a target, make builds the first target declared. This is traditionally "all", and it's common to define "all:" as the first target in a Makefile (it doesn't need a recipe--a bare "all:" is okay; you can fully define it later in the file). Alternatively, GNU Make supports the ".DEFAULT_GOAL" variable, which you can define anywhere to set the implicit target.

Somewhat confusingly, there's also a special target, ".DEFAULT", which you can define as a catchall; it's run for any requested target that has no definition of its own.