X++ is the AX programming language, and is most similar to the programming language C#.
Here is an interesting MSDN article that compares the two: X++, C# Comparison: Hello World [AX 2012]
There are several other comparison articles just like this that compare loops, string case and delimiters, array syntax, operators, and several other programming language fundamentals. You can navigate to those through the link above.
X++ is only used in Microsoft Dynamics AX. It is a class-based, object-oriented programming language. It also allows for the use of SQL queries within the language.
Also, if you are so inclined, please check out this wiki on X++ syntax: https://ax.help.dynamics.com/en/wiki/x-syntax/.
The wiki is actually very helpful and should you opt to learn the programming language in AX and become an AX developer, it will provide you with the basic knowledge that you need to proceed in working with X++.