How do you use the MC, MR, MS, and M+ button on a calculator?

These buttons work with the calculator's memory, which stores one number: 

MC = Memory Clear sets the memory to 0 
MR = Memory Recall uses the number in memory, acts as if you had keyed in that number yourself 
MS = Memory Store puts the number on the display into the memory 
M+ = Memory Add takes the number on the display, adds it to the memory, and puts the result into memory 

The buttons can be handy for doing repeated calculations with a single number. For instance, if you wanted to multiply a bunch of numbers by pi, you could key in the following: 

3.14159.. MS (stores the number) 
4 x MR = (gives you 4 times pi) 
25 x 25 x MR = (gives you 25x25 times pi) 

The M+ button can be handy for figuring out complicated expressions if you don't happen to have a scientific calculator. For example, to calculate (5 x 6) + (12 x 2) + (3 x 7), you can do the following: 

5 x 6 = (calculator says 30) 
MS (stores 30 in memory) 
12 x 2 = (calculator says 24) 
M+ (takes 24, adds it to 30, stores result 54 in memory) 
3 x 7 = (21) 
M+ (takes 54, adds 21, stores the result 75) 
MR (displays the result 75)