What is a Spike in Scrum

This question comes up time and time again and the Spike is often confused with the Tracer bullet. Adam Sroka posted a great explanation of the difference between the two on the Yahoo Scrumdevelopment group . So I am quoting Adam verbatim here - thanks Adam.

"The Pragmatic Programmers described something called a "Tracer Bullet" which:

1) Is an experimental solution that cuts through all the "layers" of
the architecture.

2) Is not necessarily time-boxed.

3) Is not intended to be thrown away.

Eric Evans talks about "Thin, vertical slices," which are the same as
Tracer Bullets.

A Spike Solution:

1) Is an experimental solution that cuts through all the "layers."

2) Is necessarily time-boxed.

3) Is always intended to be thrown away.

The reason for the distinction, IMO, is that the "tracer bullet" or
"thin, vertical slice" model is how XP teams normally work. A Spike is
an exceptional way of working when we feel we don't have enough
information to give the customer realistic expectations. The goal of
the Spike is to establish those expectations."

Spikes are a really good way for teams to figure out stuff that they don't know and need to know in order to understand the complexity so that it can be properly estimated, or quoted on or simply to find out if something is technically possible or not.