Basically there is every variation under the sun possibly but I have structured my answer so that the more common variations are earlier.
On average a woman loses around 30-40 ml of fluid, this is a combination of mucous, secretions, endometrial tissue and around 50% blood. Because of this it is a slightly darker colour than just regular old blood. This of course varies with women with blood disorders or taking medication to "thin" the blood bleeding a lot more. The second day is typically the worst. On the worst day a woman may use 3-6 pads but this can be substantially more and changing every 2-3 hours is not uncommon. It is most typically more or less constant with some worse moments in the day, but there is a huge variation.
Clots are quite common but not all women get them. They're typically more common in older females and may look just like tissue. The size of these clots can very from the size of peas to grapes and even larger than 6-7cm in some women. This is a little more worrying as this may cause anaemia or iron deficiency unlike the regular bleeding above which typically doesn't. Women affected with blood clots are much more likely to have pain.
Pain itself is more common in younger women. However usually this settles down and usually it is not painful enough to affect daily activities. In some women however they get dysmennorrhea which basically means painful uterine cramps. This is because the blood flow to the uterus is restricted due to the hormone balance and the muscle is contracting really powerfully. The lack of blood flow causes the muscle to get starved of oxygen and the pain is said to be similar to angina or like a heart attack (both being caused by a similar mechanism).
Menorrhagia is the medical term for heavy periods. This is characterised by more than 80 ml of fluid each month and affects 1 in 10 women. Usually this is diagnosed by the number of pads and tampons used. Women affected by this may require to double protect, use both a tampon and pad. They may also take a number of extra underwear or trousers to work. The symptom affecting these women is termed flooding. Unexpectedly they get a "flood" which just basically goes everywhere and is uncontrollable and is usually quite a lot of volume. This is sometimes due to an underlying disease, of which, is most commonly fibroids but may also be contraceptive devices that go into the womb/uterus, blood thinners or polycystic ovary syndrome.
There is however much variation. Some women will have only 10ml or so of blood (light periods) and that's normal for them. Some women may have irregular bleeding lasting all month. Other women may have longer periods such as 10 or even 20 days per cycle. And then cycles themselves can vary from short (20 or so days) to very long with only a few cycles per year. Basically there is every variation possible under the sun.