Good post, gunit0072003, here's some additional info:
10bT only uses 2 pairs, but 100 and 1000bT use all 4 pairs. Also, beware, many people consistently wire up their connectors incorrectly (for example, they get the pin numbers backwards). Most of the time this works, but if you cut the cable and wire up the cut ends "correctly," it won't work. So if you're reusing cable (turning a 100' into two 50', for example), make sure that the existing connectors are wired correctly.

Oh, and no post about rj45 ethernet is complete without ascii art:
Code:
 _________________
|                 |\
| 8 7 6 5 4 3 2 1 | \
|_______    ______|  \
 \      |__|\      \  |
  \      \  \\      \ |
   \______\  \\______\|
           \__\|  \
              \    \
               \    \
                \    )  <-- wire
                 \__/