A constant is something whose value does not change. For example, the acceleration due to gravity is a constant.
A variable is a part of an equation whose value can change. For example, in the equation Force = Mass*Acceleration, all of these are variables (unless their value is stated). The same equation applies in all cases, but the values for force, mass, and acceleration can be different.
Constant means unchanged. Variables can be independent (manipulated) and dependent (measured).
In an experiment, the constants are the factors you don't change from trial to trial: pressure, temperature, etc. The variables come in two flavors. The independent ones are the variables we change. We want to know what effect changing this variable has. (For instance, using butter versus oil in a cake.) The dependent variables are the ones we measure. These are the effects. (The moistness, fluffiness, and overall deliciousness of the cake, in our cake example, would be the dependent variables.)
A variable is a symbol that takes on different values, where value is a number.
For instance x is a variable that could be a real or a complex number.
Thus if x is the variable and has the value 1, then 2x + 3 = 2*1 + 3 = 5, has the value 5.
A constant is a symbol whose value is knew.
Terms are the numbers that are added or subtracted.