Dan,

You've got it right, at least the way that most of us are taught. But
let me clarify something that we aren't typically taught.
Aerodynamically speaking, there is no difference between a forward and
a side slip. One is used during final for crosswind landings in order
to align the fuselage with the runway throughout the final leg of you
pattern. However, it is, in term of how you use the controls, exactly
the same as any other slip. The difference is that you are executing
the slip along with a crab. That is, if you exit the slip, you will
have a crab angle that aligns your track with the runway.

The only reason I bring this up is that I've seen a thread that
suggests that there is a difference between a forward and side slip.
That the forward slip (by tilting the lift vector) somehow compensates
for a crosswind. Of course, it does not. Only crab angle compensates
for crosswind with respect to the ground. The slip simply aligns the
fuselage with the runway, at the cost of some glide performance.

This technique is very useful for smooth crosswind landings in high
wing taildraggers, where there is an advantage to setting one main
gear on the ground rather than both at once. It is not nearly so
applicable to gliders, though many of us were taught to compensate for
crosswinds this way. It has some minor advantages... the upwind wing
is "low" at touchdown, less finesse is required with the rudder... but
also has some disadvantages... the controls are crossed, the airspeed
indicator is unreliable, the glider will not achieve maximum
performance if sink or turbulence is encountered, there is some risk
of touching a wingtip if the flair is not properly executed.

But most important, many pilots seem not to realize that they are, in
fact, using the slip not to compensate for crosswind, but to align the
nose with the runway in order to gain a more "normal" view of the
approach. That is its only real value. Giving a slip different names
based on its application only perpetuates the confusion. And I guess
that's my point.

To sum up:

A slip is aerodynamically the same, regardless of wind.

A slip always increases drag.

A slip can be used to align the nose with the runway during a
crosswind approach (match heading with track), at the cost of
additional drag.

A tangent to the orignal thread, more concerned with semantics than
application, but I thought I'd turn the discussion in this direction.

Cheers.