There is a gasket between the tank and the bowl. There are also gaskets around the heads/nuts of the bolts that secure the tank to the bowl. (Assuming you have a two piece toilet)
If water is dripping all the time, and the tanks refills itself sometimes and you can't see the water rippling in the bowl it's probably the washers around the bolts that hold the tank to the bowl. If it only leaks right after you flush it's probably the tank to bowl gasket.
Either way, toilets are really simple...
Rebuilding a toilet takes about $20 and 30 minutes (45 for a novice).
Replace the supply line, the float valve, tanks seal and tank bolts. Pay careful attention to how the tank bolts and their washers are installed.
Kits are available at HD or any hardware store.
Learn how to do it and save money for the rest of your life.
Oh, yeah, turn off the water supply to the toilet, flush the toilet to remove as much water from the tank as you can and then sponge out the rest. Once you've got the water out of the tank, keep a towel handing and have at it.