When it comes to billiards rules, it is hard to give concrete definitions.
The fact is that there are so many different types of billiards played, that the rules for each of these games vary slightly.
Despite this, there are some general rules that are similar with most versions of this game of angles:

• Break determines stripes or solids - When a person racks up the balls and then breaks them, whichever kind of ball falls into a pocket from the break is the ball that he or she will try to hit in as the game progresses.
  For example, if a person knocks in a striped ball, his goal is to knock in the rest of the striped balls before his opponent knocks in the solids.
  This is not always true with all types of billiards, but is a general rule that is typically followed.
  
• Eight-ball is last - When shooting pool, both players usually want to avoid the eight-ball until near the end of the game.
  If the eight-ball is sunk in any of the pockets before a player has removed the rest of his balls from the table, the player who hit in the eight-ball loses.
  Even though the eight-ball is technically a solid ball, it is neutral and must be hit in by either player in order to be able to win the game.
  However it only should be struck after all other balls are gone from the table.
  
• A make means another turn - If a person makes a shot, this means that he or she is allowed to continue to shoot until he or she fails to sink one of his balls.
  According to billiards rules, a person could complete a whole game without the opponent actually ever getting a turn, although this is rare.
  
• Unscrewing is a concession - One of the rules that serious players know about and abide by that many others have not heard of is the rule that states that a person who begins to unscrew his jointed pool stick during his opponents turn is signaling that he concedes the match.
  In tournament play, a concession is equivalent to a loss.
  
• Outside interference - In the case where there is some kind of outside interference, the referee will take the balls and place them back in the same places where they stood before the shot was taken.
  Outside interference could include somebody bumping into the pool stick or something being thrown onto the pool table that disrupts the shot.
  
• Settling balls - Sometimes a ball will move by itself in between shots thanks to a small imperfection on the playing surface.
  If this settling of a ball does not alter the position of other balls, then there is nothing that needs to be done.
  However, if a ball is on the corner of a pocket and settles so that it falls into the pocket, the referee will remove the ball and reset it to its previous position.
  

Of course, these are just a sampling of some of the general rules in billiards.
When people are just playing around, these rules are not always closely adhered to.
However, if a person is seeking to strictly follow billiards rules, these rules are important to know.