Alt coin trading - Solid API Trading programs

[RockHound]

My mate's properly into alt coin trading.

Can someone explain to us what an API is and can we use it for automatic trading?

If so can someone recommend the best/very good low-cost trading platform to use?

Many thanks in advance  

He uses Cryptsy, Poloniex, Bittrex

[daybyter]

An API is an interface of an exchange to be used by external applications. So these applications can execute commands on this exchange, like 'get my funds', 'get the current orderbook', 'create a new order to sell/buy' etc.