What is a sense amplifier and write enable?

In Howstuffworks they write that memory cells alone would be worthless without some way to get information in and out of them. So the memory cells have a whole support infrastructure of other specialized circuits. These circuits perform functions such as:

• Identifying each row and column (row address select and column address select)

• Keeping track of the refresh sequence (counter)

• Reading and restoring the signal from a cell (sense amplifier)

• Telling a cell whether it should take a charge or not (write enable)

http://computer.howstuffworks.com/ram1.htm

What they don't explain is what a sense amplifier and write enable are.

Could anyone explain me in more detail what is a sense amplifier and what is a write enable?

Edited August 21, 200916 yr by Uri