Typically most people dont require the use of R/W. Most of the time you will only be writing. I personally have not really found a need to read anything back from the lcd, so my R/W is always grounded.
RS is as the name suggests. Register Select. Its used to switch between certain registers. You need it to inialize, write data, create custom characters etc...
Enable is like a 'switch' that basically says, "do something now". In order for the lcd to respond, it has to go high, delay for a certain amount of time, and then go low. Only then does it perform some function (depending on whats D7-D0)