In a linear equation the highest degree of the variable is 1. For example
3x + 1 = 2 is linear ( x is of degree 1).
The graph of the function will be a straight line.
A quadratic equation will have degree 2 ( eg x^2) and its graph will be curved . Example 3x^2 - x + 7 = 0
Simultaneous equations are 2 or more equations with 2 or more variables. For example
2x + y = 3
x - y = 1
They may be both linear, quadratic or or a mixture of both.