There are four simplest equations of a parabola which are called its standard forms. They are:
 `y^2 = 4ax`
 `y^2 = 4ax`
 `x^2 = 4ay`
 `x^2 = 4ay`
These are called the standard forms because they represent the simplest possible graphs of parabolas. In each of these equations, `x` and `y` represents a variable and `a` can be any number.
For example, `y^2 = 4ax` represents a right handed parabola as follows:

`y^2 = 4ax` 
`y^2 = 4ax` represents a left handed parabola:

`y^2 = 4ax` 
`x^2 = 4ay` represents a upward facing parabola:

`x^2 = 4ay` 
`x^2 = 4ay` represents a upsidedown or inverted parabola:

`x^2 = 4ay` 
As you can see these are the simplest kinds of graphs of a parabola possible. So they are termed as standard forms.
These standard forms help determine the various properties of a parabola. For example, for `y = 2x^2 + 3x + 1`, though it does not match with any of the standard forms above, can be easily converted to match any one of them (It can be rewritten as `y = 2(x + 3/4)^2  1/8`, which is comparable with the standard form `x^2 = 4ay`  you can learn more about converting a parabola to the above four standard forms here). Its characteristics can be determined by comparing it to the standard form.
Each standard form of a parabola has its own set of properties or characteristics, such as the coordinates of
focus,
vertex, the equation of
directrix, etc. These are discussed in
this post.