If you a need, you'll know what you need it for.
A plunge router has a movable base that allows for, well plunge-cutting,
while a fixed based doesn't. For hand use, the plunge base is often
handy -- think making a stopped cut in the surface of a workpiece w/
vertical entrance/exit. If used in a table, the plunge feature isn't of
much use although many use one as double duty.
Also, the old adage "you get what you pay for" is as true for routers as any
other power tool. Buying a cheap router, with a sloppy depth lock, will
make you not want to own one any more. You will also ruin expensive wood.
As Bob's link indicates, the router is a very versatile tool. Even a good,
capable machine might be your smallest investment. There are lots of
accessories that allow you to do a variety of operations.
Fixed base router is set for a specific depth of cut. A plunge router
can have the depth of cut changed during operation. Normally you set
the plunge router for a maximum depth of cut, start it with the cutter
above the surface and plunge it into the timber for stopped grooves.
A router is used for edge treatment of timber (roundovers, ogee etc),
trimming edges flush to sides, cutting rebates, trenches etc.
There are plenty of books on using a router. Try your local library.
A router table can be very useful for making your own moldings. It is
not necessary to go with a very expensive table initially.
I agree with what's been posted so far - get a nice router (at least
$100, Bosch, Porter-Cable, Dewalt, etc) with a 1/2" collet. Pat
Warner's site is helpful, and I'd also recommend looking at general
woodworking books, or books specific to router use.
One useful accessory to think about is an offset sub-base - I think
these are recommended and maybe even sold at Pat Warner's site. I have
one from Rockler that was designed for use as an edge guide, which I've
found very handy.
Have fun routing,
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