Creatine is the generic compound. Monohydrate, Ethyl Ester, and so on are forms of creatine. The standard and simplest form is monohydrate. This form is the oldest and has the most studies for effectiveness behind it. For some though, it causes bloating and other discomfort, so they prefer other forms. The ester form makes for better absorption, although some claim it is not as effective as the monohydrate form. There are also other advanced creatine forms such as malate, gluconate, alpha ketoglutarate and so on. They are all basic creatine bonded with other compounds to produce different levels of bioavailability and effectiveness.
If you try the monohydrate form and do not get bloats from it, you may want to stick with it. Dosage is usually about 3g daily pre-workout. On non-workout days, too. On an empty stomach. You may want to cycle it.