A battery, also called a battery pack, is what powers an electric RC, and what powers the electronics on a nitro or gas RC. A battery is a group of cells, but often a cell is called a battery also.
In the past, hard core racers directly soldered batteries to the speed control in electric RCs, with no connectors. This is called hard wiring, and is sometimes still done, but there are now connectors available that have the same or less resistance as a piece of 12 gauge wire the same length as the connector.
Matched batteries[]
Matched GP3300 battery
A matched battery pack is one which has been processed through a special matching process, which ensures that all the batteries in the pack are close to the same quality. Usually stickers with the stats are applied to each cell. For performance reasons, the battery is usually soldered together by hand, but matched manufactured batteries are now available. Matched batteries are available pre-built, or you can buy a package of matched cells and do it yourself.
Soldered batteries[]
For performance reasons, a battery can be soldered together. On an electric RC this is normally done in a side-by-side flat configuration. A large, low resistance battery bar connects each cell. A shrinkwrap is often applied over the individual cells to protect the labels, but the entire pack is almost never shrinkwrapped. This means it is fairly easy to short the pack out, releasing the gross, smelly, poisonous liquid contents of the battery.
Manufactured batteries[]
These batteries, normally in a stick configuration, are spot welded with tiny sheet metal tabs. This makes the battery much cheaper, but causes a loss of performance. They are always completely covered in shrink wrap, and sometimes the cells are enclosed in two cardboard tubes. This means it is more difficult to completely change the wires, but protects the battery from shorting out. You can get slightly more performance out of this type of battery by dissassembling it and soldering it.