An equation between two variables and its shows a straight line when plotted on a graph. e.g., x + y = 4.

An equation may have linear expressions on both sides. e.g., x + y = 4, 2x + 4 = 10, 5x = 10

An equation in which only one variable is use i.e., linear equations in one variable. e.g., 2x + 5 = 7.

The uses of linear equations is in their many applications; different problems on numbers, ages, perimeters, combination of currency notes, and so on can be solved using linear equations.

A variable is a number which is use in alphanumeric character. e.g., x, y, a, b etc.
A non-variable is a number which is use in numeric values. e.g., 2, 3, 10 etc.

An algebraic equation is an equality involving variables. It has an equality sign.
The expression on the left of the equality sign is the Left Hand Side (LHS).
The expression on the right of the equality sign is the Right Hand Side (RHS).
e.g., 3x + 5 = 9 is the equation. Here, 'x' is the variable, '+' is the operator, '5' and '9' are be the non-variables, '=' is the equality.