A deep-cycle battery is a lead-acid battery designed to be regularly deeply discharged using most of its capacity. Used in renewable energy systems, they are different from car batteries and that difference is critical.
RE systems by nature are cyclical: energy is captured and stored, then later consumed, in a (usually) regular. For example, in a battery-based solar electric system, the energy produced daily by the solar panels is stored in the battery bank, which is then used by loads at night or on not-so-sunny days. This repetitive process subjects the batteries to a slow, daily charge and discharge pattern.
Car batteries are not meant to be used in this way. They can release a great deal of their stored energy at once, to start the engine, and then they immediately receive a rapid recharge from the car’s alternator. They’re not meant to recover their charge slowly, as would happen in a solar electric system.