"Starcruiser" is one of those terms that's almost impossible to define.
Y'see, it's a class of ship, so it changes meaning depending on context (and universe).
In Star Wars, frex, a 'cruiser is a starship (either "heavy," "light," or "medium") that can deploy a certain amount of firepower within a certain span of time. It can also (again, depending on context) refer to any ship capable of carrying passengers in addition to its crew.
In Star Trek, by contrast, "starcruiser" seems to be just a(n unused) synonym for "starship." I've heard both the Enterprise and the Defiant referred to as "Federation starcruisers."
Most of the time, starships fall into three categories: starfighters (X-wings, slipfighters, starfuries, etc), transports (the Millennium Falcon, the Eureka Maru, etc), and capital ships (Mon Cal cruisers, High Guard battleships, Earthfroce Destroyers, etc).
Then you get into the ships that fall into two or more categories, and stretch all definitions. The Whitestar, for instance, is the size of a cap ship, but functions like a giant starfighter (blisteringly fast, fires forward-only, and looks like a predator even when standing still).
"The Beasts know much that we do not." -Ancient Jedi proverb