The Glob of Goo is a raw material used for throwing (careful with that!) and crafting.
You can throw Globs of Goo by putting them into your quickbar, select the according slot and then right-click. Only one glob from a stack of goo will be thrown whereever your crosshairs points.
Globs of Goo can directly be thrown at enemies, for example Keepas, ensnaring them in a sticky trap and preventing them from attacking for about 5 seconds. Goo-blots from thrown Globs of Goo are not sticky for player-characters, but bouncy - you can use them as spring boards until they will vanish by themselves after a few seconds. Once thrown, the Goo cannot be picked up again nor rotated.
Warning: be careful not to throw Globs of Goo against any usable objects like at doors (into doorways), storage chests, crafting stations, teleporters or the like. There is a known bug that can make Goo-blots permanent in this case or can even make usable objects vanish. In such ways whole chests - together with all their content - can disappear.
Since R17 Goo can also be crafted into elastic Goo Blocks (green, orange and purple). The first recipe will be unlocked after taking Goo for the first time, two more after crafting the first one in your Crafting Menu (default key "Q") or at any Crafting Table (that still exist in worlds created before R22 on September 16th 2015).