WHAT IS MQL?
MQL – Meta Query Language – is the computer language used to write Expert Advisors.  The syntax and rules of the language are similar to the C programming language.
The first release of MQL was 3.0.  The 3.0 language was more of a scripting language.  It was not easy to create structured programs with 3.0.
Next 4.0 was released. This is currently the most popular version.  It is very powerful and easily allows functions to be created and fully supports structured programming. 
The latest release is 5.0.  5.0 introduced the concepts of object-oriented programming to MQL, as well as backend upgrades to make it easier for brokers to support MetaTrader.
You are probably not familiar with these terms, such as scripting language, structured programming and object-oriented programming.  
And that’s OK!  These are terms for computer geeks (like me).  You don’t need to know these terms to build profitable Expert Advisors – that’s what the Visual Traders Studio is all about.