'Heat' or 'season' is the time when a bitch is ready for mating and when pregnancy may result. The first 'heat' occurs at 6 to 9 months of ago, and the average bitch comes on 'heat' twice yearly.
During the first stage of 'heat' a bitch prepares for mating and her vulva swells, discharging blood-stained fluid. She becomes attractive to dogs, and her temperament may be affected at this time - this period may last for approximately 7-10 days. The bitch will then be receptive to the male for a further 7-10 days period, so it is advisable to isolate the bitch for three weeks during her season.
Problems that a bitch in 'heat' can pose (e.g. when going on holiday) are best discussed with a vet well in advance; suitable medication may be prescribed to postpone the season.
If you have no intention of ever breeding from your bitch you may wish to have her spayed - the benfits of spaying can be explained to you by your vet or a practice nurse.
Likewise, you may have a male dog whom you wish to have neutered. Again, your vet or a practice nurse will be happy to discuss the benefits of this procedure with you.