Broadly speaking, there are two types of trading account you may be interested in.
Regulation T Margin, or Cash.
Reg T Margin gives you the facility to partially cover a trade on money 'borrowed' from the Broker; Cash means that you need sufficient funding and reserves to cover your trades.
In either case, if things get out of hand against you, the broker will demand that you top up your account immediately, or may just close your positions without notice. This is typically known as a 'margin call'.
These links may help: