Presumably wireless. Sometimes a combination of poor reception or interference with wireless security, especially WPA.
I have this frequently when using laptop in marginal-coverage areas at home.
First check that you're okay when connected by wire to the router instead of wireless.
Then, try turning off wireless security in the router before going back to wireless and your usual location temporarily.
If the disconnections stop it could be that wireless security is the sole cause so check that you have the right type selected in both router and laptop.
Check wireless mode in the router matches the laptop's mode (n, g, mixed mode).
You then may need to tackle the issues of wireless reception and interference thus:
1) raise the router above furniture level
2) Experiment with channels (some will work better or worse depending on your environment)
3) If you can detect strong neighbouring wifi, use a channel 5 stops away from strongest.
4) Relocate cordless phone base or video sender etc.
5) Be prepared to move the computer (or at least turn it so your body is not between the router signal and the wireless adapter's antenna.