Friction shifters come in one of two basic designs.  One is a smooth pure friction shifter, the other is commonly referred to as a Retro-Friction or ratcheting design.  Pure friction shifters utilize a set of flat pressure disks that slide smoothly against one another as the shifter is operated.  These disks are under enough pressure that the strength of the rear derailleur's spring alone is not enough to move it.  Friction shifters are known to loosen up slowly during use, and sometimes when paired with frames that are particularly flexy the flex of the bottom bracket can cause the shifter to slowly loose its position.  Retro-Friction shifters operate in a similar way to pure friction shifters as there is no distinct positions per shift, however they do have a ratcheting feel when operated in one (or sometimes both) directions.  These ratchet designs allowed for a lighter feel when shifting in the direction of the ratchet since you typically only feel the resistance of the derailleur spring instead of both the spring in addition to the friction of the shifter's disks.

It is important to note that nearly all index style shifters (not including integrated shift-braking levers) included a Friction mode of some sort.  It was not till later Campagnolo Syncro II shifters and the introduction of STI / Ergo shifting systems that a pure friction mode was abandoned on shifters.