What happen when a female dog is in heat?

What are the signs of dog in heat?
Can the dog breed when not in heat?
Are dogs in heat agressive?
How long does the heat last for?
Answers:
Sexually mature female dogs will go through a heat cycle once, or more commonly, twice a year. Each cycle consists of four stages:
1. Proestrus: Characterized by swelling of the vulva and release of a bloody vaginal discharge. Sometimes the discharge is milky and other times dark red (or somewhere in between in colour). Between bitches, there is a large variation in the length of proestrus, with an average length of 9 days, and a range of 1 to 17 days. This is the "pre-heat" during which the male is progressively more attracted to the female, but the female still rejects the male. This is a high female hormone (estrogen) stage.
2. Estrus: This phase is often termed standing heat because it is during this stage that the female will receive the male dog. Average length for estrus is 9 days, but again, between dogs there is large variation (range 3 to 21 days). At this point, the discharge may be absent, or perhaps straw coloured. This stage is characterized by an increase in progesterone hormone and an LH (luteinizing hormone) surge that leads to ovulation.
3. Diestrus: At this point, the female usually no longer stands for the male, especially further into the diestrus stage. Vaginal discharge tapers off and the diestrus phase lasts an average of about 60 days. Diestrus begins 6 days after ovulation. Progesterone hormones remain high throughout this phase.
4. Anestrus: This is the resting phase between the active portions of the cycles. It normally lasts about 5 months.
Hopefully that answers all of your questions. If not surely one of the sources that I listed below can help you out.

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their volva swells up, and they have bloody discharge for about a week. (they come into heat 2-3 times a year)
dogs are fertile from a few days before they start to spot, untill a few days after they finish spotting
MY dogs are not agressive. Dogs CaN become, but if they are normally nice dogs, then they woun't be agressive.
if you are planning to breed, ask your vet when the best time to breed your dog is. he should be able to tell you the exact days top breed your dog.

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When a female dog is in heat that means it is old enough to produce puppies.
you can tell when the female dog is in heat when it
* Show a change in mood shortly before her season commences, acting a bit touchy.
* Sometimes, but not always, the breasts will swell slightly. This can also be a sign of a phantom pregnancy, when a female may begin to show signs of being pregnant even if she's not, so watch if this happens, but it usually resolves itself in a few weeks after the cycle ends.
* Males are great early warning detectors and can smell the change in a female's hormones before she fully comes into heat.
* The vulva can show some swelling, but it is quite variable, some girls hardly swell at all, while others swell up like a golf ball.
* Tail flagging is another sign. When a female is ready to be bred, she'll usually stand quite still while the male investigates her vulva, holding her own tail up and wagging it side to side to make sure he gets the scent.
* Blood discharge is usually the biggest indicator, with a pinkish red colored discharge the first week, usually turning to a tannish color during the fertile period, and changing back to a reddish color before gradually stopping altogether. Some females keep themselves extremely clean, and it may be difficult to tell if they are in season at all.
No, it cannot breed when not in heat.
You have to wait at least the 16th day if you want to breed it.
Yes dogs in heat do get agressive.
A female dog is in heat for approximately 21 days. In most dogs here is obvious vaginal bleeding for about 10-14 days. Ovulation occurs after that and female dogs are most receptive to being bread for the 7-10 days after bleeding stops