Algorithmic Trading is a used to describe a number of concepts to trade automatically using computer systems and rules.
Trading that is controlled by a computer or computer program is gaining in volume and use. What is actually meant by Algorithmic Trading depends on the context and use.
It can refer to automated trading systems. Such as a black box trading system that is trend following.
It can refer to order systems. Such as the order system that splits out a large order into smaller pieces.
It can also refer to High Frequency trading.