The combined pill is taken every day. It may not be reliable if you miss a pill. You should talk to your doctor about the impact of missed pills for the particular pill you are taking.
The mini pill must be taken every day but is not reliable if taken over 3 hours late.
The main way the combined pill prevents pregnancy is by preventing eggs from leaving the ovaries.
The minipill prevents pregnancy by changing the mucus at the entrance of the womb (uterus) to restrict the sperm from reaching the egg. In some women it stops the release of an egg each month.
Return to pre-existing fertility is likely to occur very quickly after ceasing to take the pill.