Of course, in Korea, like WhatisProtoss said, Starleague refers to OSL, as OGN originally came up with the name "Starleague". Remember, MBC's orginial name for it was KPGA Tour, and since that wasn't working too well they stole the name and called it MBCGame Starleague.
So, Starleague is SUPPOSE to refer to OSL only, and MSL is called MSL. (interestingly enough, the MSL commentators refer to OSL as "the other league" they NEVER say OSL or OGN Starleague, while they call themselves MSL, and OSL calls themselves "Starleague")
But like I said, I, and most foreigners, refer to any major league as a starleague.
I wouldn't agree with Whatisprotoss's comment about MSL being a minor league though. The OSL is considered higher because of the prize-money difference, but I think they give the same amount points? I'm not too sure about that.