A Geothermal Generator works on the same basic principle as normal Generators do.
Geothermal generators use the heat of Lava to generate electricity. With the release of IC², the generators accept both Lava Buckets and Lava Cells. The latter are stackable and thus require less maintenance, but cost Tin to produce.
Note for those using Buildcraft: the Geothermal Generator will now connect to Buildcraft's Waterproof Pipes, and will receive lava carried by them. It will also accept lava directly from an adjacent Buildcraft Pump.
As with all Generators, the lava-based fuel of choice is inserted into the lower slot, and optionally an RE Battery in the upper slot. The Geothermal Generator will charge the battery or output power at a rate of 20 EU/t.
A single Geothermal Generator can store 24 units of lava - thus, inserting a full stack of Lava Cells will instantly use up 24 of them and fill out the lava gauge. Unlike the furnace, the Geothermal Generator does not consume the bucket when a lava bucket is used as fuel. It is returned to your inventory.
Storage: 480,000 EU
Input: Lava Buckets or Lava Cells
Output: 20000 EU per Lava Bucket or Lava Cell at a rate of 20 EU/t
If you have the BuildCraft mod installed, you can effectively generate infinite power. Simply connect one (or more) of these magical machines to a BuildCraft pump pumping from a lava lake in the Nether.
Also, if you have Eloraam's RedPower installed, you can pump the lava from the volcano** into the generator.
* Not responsible for limitations due to laws of physics.
** Not quite as "infinite" as the Nether.