Shapers are usually broken down into two categories: spindle mounted on the side and spindle mounted at the base of the table. In the latter, which is the more common model, the wood is cut from the bottom.
Shapers work by mounting special bits onto its spindle. The spindle rotates at moderate speeds and cuts a profile into the woodpiece as it's fed through. Some spindles can mount router bits, but most use specially fabricated shaper bits for the bigger jobs that routers can't handle. The most common spindle sizes are 3/4" for small shapers and 1 1/4" for larger ones. Some shapers have interchangeable spindles, but these are usually high-end, with reversible and tilting capabilities.
Advantages of a Shaper
* Can handle a larger volume of work than a router can.
* Can carve on the side or the bottom.
* Create specialty cuts with various accessories.
* Can hold multiple cutter heads simultaneously for quick changes between tools.
Disadvantages of a Shaper
* Large, heavy-duty, and more than the beginning craftsperson needs.
* Since they are fixed in table mode, they do not offer the versatility of a router.
If you're looking for something smaller, lighter or less expensive, you might want to consider a router instead.