Answer: An equation MUST contain an equal sign. Others, you can multiply/divide/add/subtract two variables or even keep a single variable.
Generally most common equations include variable/variables, constant and a co-efficient.
Variable: a symbol (usually a letter) standing in for an unknown numerical value in an equation. Commonly used variables include x and y .
Constant: a value or number that never changes in expression. It is constantly the same.
Coefficient: a number that is being multiplied by the variable.
For example:
2a - 6y + 14 = 10 is an equation.
Here the variables are, "a" and "y". Constant is, "14" and coefficients are, "2" and "6".
More,
an equation could be very simple as well. There can be no coefficient's or variables in a equation.
For example:
10/5 = 2
There are no variables or coefficient's or constants in the above equation.
If any expression has an equal sign, forgetting everything, we can say for sure that it is an equation. However if there is no equal sign then we call it an expression. From the above example, if it was only " 10 / 5 " then it would be an expression.