To add a bit more detail to the 'new ball' issue:
A new ball is initially highly polished on both sides. It will move through the air quickly (which favours fast bowlers) but it keeps to fairly straight line (which also favours batsman at the same time, as it's easier for them to 'pick up' the flight of the ball).
However the fielding side will have agreed beforehand as to which side of the ball will be polished at every opportunity. (Cricket balls never have the same design embossed into both sides, for that very reason). While it's against the rules for the fielders to deliberately roughen the other side, it's amazing how often a fielder will 'accidentally' drag the rougher side along the ground slightly as he picks it up! So now there's a ball with one shiny side and one rough one. That will move slightly slower through the air but the difference in air resistance (together with wrist movement from the bowler) means that it will start to 'swing'. (The amount of swing depends upon temperature and humidity).
As the ball gets more worn, it's harder to keep one side effectively polished (so the fast bowlers aren't as effective) but the seam will start to lift, meaning that the ball is no longer circular and, with the help of a good (slow) spin bowler, likely to leave the pitch at a sharply different angle to that on which it landed.
When the fielding side becomes entitled to take a new ball, their captain isn't obliged to do so. If the weather (and the expertise of his bowlers) favours spinners, he'll keep the old ball but, if he wants to terrify 'tail enders' (by having his fast bowlers send a very hard object directly towards them at 90mph) he'll take the new ball.