Bruises also called contusions develop when small blood vessels under the skin tear or rupture, most often from a bump or fall. A hematoma is similar, but more concentrated giving the skin a spongy, rubbery, lumpy feel.
They can also be caused by a muscle pulls or muscle tears, Vigorous exercise, or an leaking aneurysm (balloon-like bulge of a blood vessel).

Bruises are the second most common sports injury after strains, and although not considered a serious injury, they can cause mild discomfort and create quite a nasty looking discoloration on the skin.

Bruises change colors over time in a predictable pattern, so that it is possible to estimate when an injury occurred by the color of the bruise. Initially, a bruise will be reddish, the color of the blood under the skin. After one to two days, the red blood cells begin to break down, and the bruise will darken to a blue or purplish color. This fades to green at about day six. Around the eighth or ninth day, the skin over the bruised area will have a brown or yellowish appearance, and it will gradually diminish back to its normal color.

Special conditions:
Bruises also develop easily in the elderly, because the skin and blood vessels have a tendency to become t fragile with aging. In the condition known as purpura senilis, the elderly develop bruises from minimal contact that may take up to several months to completely heal.
People with vitamin C deficiencies bruise more easily than others.

The use of nonsteroidal anti-inflammatories such as ibuprofen (Advil) and naproxen (Aleve) may lead to increased bruising. Aspirin, antidepressants, asthma medications, and cortisone medications also have this effect. The anti-clotting medications also known as blood thinners, especially the drug warfarin (Coumadin) also increase risk of bruising.
Sources: Much of the above is taken from Bruises at Encyclopedia.com
See Links below for other sources./

Bruise treatment that lessens hemorrhaging (RICE):

• Rest
• Ice - Start applying ice or cold packs as soon as possible This also decreases blood vessel hemorrhaging and immune response causing edema (accumulation of fluid beneath the skin,)
• Compression
• Elevation

Do not use non-steroidal anti-inflammatory drugs (NSAIDs) [aspirin and ibuprofen (Advil or Motrin), naproxen (Aleve or Naprosyn) or ketoprofen (Actron or Orudis) ] for the first 24 hours after an injury that caused a bruise. NSAIDs may prolong bleeding under the skin.

After 3 days:

• Heat
• Moderate exercise
• Massage
• Drink plenty of fluid
• Topical applications are usually recommend to speed healing and to reduce the pain associated with bruises. Vitamin K, Astringent herbs such as witch hazel , Hamamelis virginiana, or the homeopathic remedy, Arnica montana.
  Applying shredded raw potato directly to the injury for about an hour has also been recommended.
• Diet: Calcium and magnesium are helpful in healing bruises;

You will see a range of recommendations as to when to start applying heat.