There are three main types of lead-acid batteries:
Wet cell batteries use a liquid electrolyte in an unsealed battery container. They’re the cheapest type of battery, but require maintenance, and careful placement to ensure ventilation. While charging, these unsealed batteries will release hydrogen gas (highly explosive!) so must be installed in a well-vented location, outside of the main living quarters. If installed inside, they must be placed into a proper battery box with sufficient ventilation to the outside.
Gel cell batteries use a thickening agent to keep the electrolyte from sloshing. They’re a sealed type battery, meaning the electrolyte cannot be topped off with water if required (such as after overcharging), so careful attention must be paid to setting proper voltages. In addition, they’re not able to accept as high a voltage as wet cells or AGMs, so make sure that your charger is calibrated for gel cell batteries.