In the US, Class B airspace is found around the largest airports, and is usually layered "like an upside-down wedding cake" - e.g. 0-8000' out to 10DME, 2000-8000 to 12DME, 4000'-8000' to 20DME etc. AOPA has a pretty picture
38.200.9.137/class_b.html
To enter class B airspace you need a Mode C transponder, radio & ATC clearance, and it's often quite easy to transit the outer areas VFR.
[ Edited for URL typo ]
[ 21 September 2001: Message edited by: FrontWindowSeat ]