Marshrutki are privately-owned vans that follow set routes between subway stops and other destinations. They basically offer an alternative to bus routes that might not be as direct or tend to be infrequent. They're more expensive than buses, but can be quite convenient. I think they cost about a dollar these days.
The catch? It's quite an experience! People tend to PILE IN. If you're stuck in the back, it can be a rather smelly ride. While smoking is frowned upon, it's not prohibited... Payment is on the honor system and I doubt anyone has ever gotten away without paying... People are VERY much into others' business when squeezed in there and an angry babushka would assault you. One passenger sometimes appoints him/herself the fare collector, or you pass your money to the driver. Be prepared for wrath if all you have is a large note and expect change!
Some vans are death traps... This one, however, is new and so darn pretty! I love all the old-fashioned Russian folk illustrations on the doors.