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Welcome to Grex, Lucie.  It gets easier.

This sounds like a good item for discussing brakes.

brakes... what about them?  I know.. for those of us with rear drum brakes..
I've heard a lot of talk about people wanting to upgrade to rear disc???  Is
there much advantage to this?  Unless you are doing in excess of 100 MPH on
a regular basis, the only thing I see rear disc brakes doing is adding weight.

Rear discs look cool.  They are easier to service and they might be cheaper
to put on the car.  That, however, has a few strings attached, litteraly.
The rear brakes are also set when you engage the parking brake.  Adding a
parking brake to a disc introduces a few gotchyas. Some of the rear disc
systems actually had a mini drum in the disc hub just for the parking brake!
Others have cams that push the disc pad up against the rotor.

I'm not sure discs are any heavier than drums.  Seems they could be lighter. 
It's certainly true that they're easier to service.  My van has drum brakes
in the rear that aren't self adjusting, and *that* is a pain in the tail. 
'Specially when the adjusting stars seize up.

The main advantage of disc brakes is  better heat dissipation.  With heavy
braking, drum brakes build up heat, and this causes 'fade' -- the brakes
don't grip as well.  Disc brakes tend to dissipate this more easily.  They
also aren't as susceptible to losing effectiveness when wet, I believe.  For
normal driving front discs and rear drums are probably plenty good enough.

Are there different kinds of bicycle brakes (other than new ones and worn out
ones, which I have)?

Yes, there are several different kinds of bicycle brakes.

such as?

Re bicycle brakes:  Center-pull and side-pull brakes.  Both use pads that
grip the wheel rim, but in side pull brakes the cable attaches to one side
and pulls one pad against the rim, and the lever action draws the other one
in.  Center pull brakes have the cable split and pull both pads in.  Center
pull brakes are more expensive, but  much more effective.  They're also
easier to adjust so they don't drag on the wheel when released, something
side-pull brakes tend to do when the pivots aren't perfectly free.

Every so often you'll see mountain bikes with disc brakes.  I imagine these
are more effective in wet conditions, when normal bike brakes are pretty
much useless.

On side pull brakes, the cable housing pulls one pad in, while the cable pulls
the other pad in.  It's just a different way of squeezing the two calipers
together.  There are also various different kind of side pull breaks,
Shimano's dual pivot stuff, for example.  In center pull brakes, there are
center pull calipers, which look like side pull brakes except that the cable
pulls on them differently, and cantilever brakes, which are two separate
pieces that attach to the frame on each side of the wheel, and then the cable
pulls the other ends of them together, causing them to grip the wheel.  Disk
brakes I've generally seen only on tandems, where it's necessary to be able
to set a brake and keep it on all the way down very long hills, to keep the
speed from getting out of control.  Regular brakes that rub on the rim can't
be used that way, since they generate enough heat to blow tires if they're
kept on too long.  There are also, of course, the coaster brakes found on very
low end bikes, and probably a few other varieties of brakes that I'm
forgetting.

Disk brakes are also nice because they are not as susceptible to dirt and
grime accumulation and because they wear down instead of your pads grinding
metal off the rims and weakening them.  Rim brakes also require a rim to 
be true and of uniform thickness.  Otherwise they rub when not braking and
pulsate when braking.  Rim brakes are light and keep the bicycle simple.
They will be difficult to displace.

I use my regular black rubber (rim) brakes all the time going down hills and
have never blown a tire.  Why would tandem bikes go down hills slower than
one-seaters?

They don't.  They can go down hills a lot faster, since there's a lot more
weight involved, but they don't corner as well, so there's a need to slow them
down.

You're probably not starting at the top of steep down hills that are a mile
or two long and riding the brakes all the way to the bottom, which is the sort
of thing that can cause big problems.  Also, it depends on what type of tires
and tubes people are using.

It's not a problem around here.  However, two adults on a tandem touring
mountainous areas need to dump a lot of energy when coming down some of
those steep, long and winding passes.  I have even managed >40 MPH on some
of the down hills in the Sleeping Bear Dune area just coasting on my
single.  Tandems have twice the mass, less wind resistance (the two riders
are in-line) all on a set of wheels and a braking system found on a single.
Brakes are serious business on tandems as dumping one while going fast
is a major pain.  On mountain bikes they make sense too, as anyone who
has ridden through the mud can imagine.

I see how dumping one rider would be a major pain, but what did you really
mean?

I was referring to axis of the tandem shifting 90 degrees as does the box
on a dump truck when it's dumping its load.

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