You should be mostly using structures.  Unions are a powerful but special case tool.

Structures are a group of variables.  They give you a way to use one name to refer to a collection of variables.  Each variable get it's own space in memory.  All the variables are packed one after another in memory. 

So for example:

struct Vec2

{

  float x;

  float y;

};

Gets a you a block of memory containing space for TWO unique floating point numbers, one for x, one for y.

A union is a group of names that point to the same block of memory.  They provide you with a way to access the same memory with different names.  This is sometimes useful.

For example:

union Vec1

{

  float x;

  float y;

};

Gets you a block of memory with space for ONE floating point number.  You can refer to the number by either x or y, but it is the SAME memory location and therefore the SAME number.

If you are asking 'why would I ever want to do that?', you're asking the right question!  :)

Unions are useful in a handful of case but generally you're going to be using structures to store your data.  My advice is to use structures and then one day you'll find yourself thinking 'what I really want to do is access this variable as either two ints or an array of 8 chars'.  When that moment happens, you'll have found your use for unions.