A deep-cycle battery is a lead-acid battery designed to be often deeply used up the majority of its capacity. On the other hand, starter batteries (e.g. most automotive batteries) are designed to provide brief, high-current outbursts for cranking the engine, so often releasing just a little part of their ability. While a deep-cycle battery may be used as a starting battery, the lower "cranking current" mean an oversize battery could be needed.
A deep-cycle battery was made to use up between 45% and 75% of its capacity, with respect to the building of the battery as well as the maker.
The difference is that their layout is optimized by the batteries:
The battery of a car was created to give an extremely great deal of current for a brief period of time. This spike of current is needed to turn around the engine during starting. The alternator supplies all of the electricity the auto wants, so a car battery may go through its whole life without being dead more than 20 percent of its own total capacity after the engine starts. Used this way, a car battery can persist for several years. To reach lots of current, thin plates are used by a car battery so that you can improve its surface area.
A deep cycle battery was created to give a constant quantity of current over an extended time. A spike can be provided by a deep cycle battery when needed, but nothing such as the spike a car battery can. A deep cycle battery can be made to be deeply fired over and over again (something that could destroy a car battery quite fast). To achieve this, thicker plates are used by a deep cycle battery.
A car battery usually has two evaluations:
CCA (Cold Cranking Amps) - How many amps the battery can generate at 32 degrees F (0 degrees C) for 30 seconds
RC (Reserve Capacity) - How many minutes while keeping its voltage that the battery can deliver 25 amps
In sum, a deep cycle battery can withstand several hundred complete discharge/recharge cycles, while a car battery isn't designed to be fully discharged.