A description block is a dependency line optionally followed by a commands block:
targets... : dependents...
commands...
A dependency line specifies one or more targets and zero or more dependents. A target must be at the start of the line. Separate targets from dependents by a colon (:); spaces or tabs are allowed. To split the line, use a backslash ( \ ) after a target or dependent. If a target does not exist, has an earlier timestamp than a dependent, or is a pseudotarget, NMAKE executes the commands. If a dependent is a target elsewhere and does not exist or is out-of-date with respect to its own dependents, NMAKE updates the dependent before updating the current dependency.
For more information: