An Expert advisor type is determined by the way it deals with the market and when and how it opens and closes positions.
Many MetaTrader 4 platform Expert Advisor types have recently invaded the market, only 4 basic types are the core of most of them.
An EA coder can create an Expert advisor depending on a pure basic type or merge more than one type in one Expert Advisor to get the advantages of both and minimize the disadvantages depending on each coder's creativity in programming and refining his product, while some types can't be merged together as logically their trading behavior can't be joined.
The 4 basic Expert Advisor types can be summarized as follows:
This EA works by opening two opposing positions and let each of them runs and watches their profitability, the winning position is left and may be supported by other similar positions, while the losing one is limited and may be closed. This EA in its pure form doesn't need to analyze the market conditions and price breakouts or financial news.
This is a sub-type of hedging Expert Advisors but is considered a separate type in its pure form, it works depending on financial news releases taking advantage of large price shifts due to news events.
This EA depends on technical analysis of the market behavior, a predefined support and resistance levels are the determinants of its trading decisions, once broken by the currency price, it opens a correlated position.
This is the most commonly used by Forex traders and the most restricted expert advisor by the Forex brokers! It can get your account heavily restricted or even closed, but professional programmers could modify such Expert Advisors to comply with certain brokers. It trades by securing many successive positions and close each of which as soon as it makes a small profit.
These are very important points that are necessary to learn about Forex Expert Advisors.