What's the difference between the signed and unsigned right shift operator in Jana?

Question

Possible Duplicate:
Bitwise shift operators. Signed and unsigned
Difference between >>> and >>

Given:

int i = -1000;

What's the difference between:

i = i >> 4;

and

i = i >>> 4;

?

This particular example isn't important, I'm just trying to understand the difference between the signed and unsigned bit shifts.