It is a phosphoester. Recall that esters can be thought of as a combination of an alcohol and an acid. In this case, the OH of the glucose acts as the alcohol and phosphoric acid acts like the acid. In this case the ester looks like:
O
||
-O-P-
Another common type of ester in biology is a thioester:
O
||
-S-C-
[Edit: thanks Kremar]