An Expert advisor type is determined by the way it deals with the market and when and how it opens and closes positions.
Many types of these MetaTrader 4 plugins have recently invaded the market, only 4 basic types are the core of most of them.
An MQL programmer can create one depending on a pure basic type or merge more than one type in one tool to get the advantages of both and minimise the disadvantages depending on each coder's creativity in programming and refining its product, while some types can not be merged together as logically their trading behaviour can not be joined.
The 4 basic types of them can be summarised as follows:
Hedge Expert Advisor
This type works by opening two opposing positions and let each of their 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. In its pure form it doesn't need to analyse the market conditions and price breakouts or financial news.
Examples of Forex Robots containing hedging element in its coding are Forex Warrior, Iris Fx. Forex Growth Bot.
News Expert Advisor
This is a sub type of the hedging. Is considered a separate type in its pure form, it works dependent on financial news releases taking advantages from large price shifts due to news events.
Examples of Forex Robots that contain a financial news analysis element, what's called News Filter are Forex Combo System v5.0, Happy Market Hours and Volatility Factor EA.
Breakout Expert Advisor
This type depends on technical analysis of the market behaviour, a predefined support and resistance levels are the determinants of its trading decisions, once broken by the currency price, it opens a correlated position.
An Example of Forex Robots that contain a breakout element in its core added to important programming configurations is Forex Robin VOL.
Scalper Expert Advisor
This is the most commonly used by Forex traders and the most restricted type by the Forex brokers! It can get your account heavily restricted or even closed. Professional programs could modify such types to comply with certain brokers. It trades by securing many successful positions and close each of which as soon as it makes a small profit.
Many examples of mainly scalping Forex Robots exist and include Fapturbo 2.0, Forex Striker, WallStreet Forex Robot and PipJet Robot.
These are very important points that are necessary to learn about those MetaTrader 4 plugins.