I have ordered StarCraft 2 from Play.com, but not received the package yet.
I just wanted to know: is multiplayer Terran-only or you can control even Protoss and Zerg?
Yes, you can play all races in multiplayer. However, there aren't all technologies you might find in campaign (which is Terran, with few missions in Protoss storyline) as it'd imbalance it.
All the three races are available to be played on multiplayer. Before you start a multiplayer match, you choose what race you want to play as.
On the campaign, however, you play solely as Terran (as the whole StarCraft 2 is going to be set on a trilogy).