Knowing the official rules avoids arguments and helps you play better. Here's the essential summary.
Your turn
On your turn you can play a word, exchange tiles (losing your turn) or pass. After playing you refill to 7 tiles.
Blanks
Blank tiles stand for any letter but score 0 points. Once placed, the chosen letter is fixed.
End of the game
The game ends when the bag is empty and a player uses all their tiles, or after several consecutive passes. Each player subtracts the value of their leftover tiles; whoever went out adds everyone else's leftovers.
On Scrabbly all of this is applied automatically — just focus on playing.