A sprain is a soft tissue injury.  It is typically caused by excessive stretch to the soft tissue.  For example, if you twist your ankle, the soft tissue that hold the ankle bones together gets stretched, and you would call it a sprain.  

Below we'll discuss some of the soft tissues that may become strained.  

Each joint consists of two bones, with soft tissues holding them together.  There are various types of soft tissues that help hold bones together--including fascia, tendons, periosteum, and capsule, but ligaments--short, cord-like structures are most directly responsible for this duty. 

Ligament injuries are quite common when you twist your ankle, but they may occur virtually anywhere.  Symptoms can range from aching discomfort to deviated or misaligned toes.  

Conservative measures like rest, ice, immobilization, and anti-inflammatory medications may help, but when instability is chronic or severe, the ligaments may be repaired surgically.    

The soft tissue covering that extends like a sheet around a joint is termed capsule.  It's the same kind of white connective tissue you'd see if you've ever pulled apart a turkey or chicken leg.  Adding the suffix -itis to the end of a word means inflammation, so it follows that capsulitis means an inflammation of the joint covering. 

Capsulitis injuries are very common in the ball of the foot.  Indeed, this is probably the most common cause of metatarsalgia.  Unfortunately, it is also one of the most overlooked diagnoses, which is too bad, because if your metatarsalgia is caused from capsulitis, it can usually be resolved fairly quickly.

When the damage is severe enough, the capsule may even rupture, usually on the bottom of the foot, destabilizing the entire joint.  Indeed, such a rupture can be a cause of a hammertoe. 

Synovitis is a condition of inflammation of the joint fluid known as synovium.  Yes, the fluid inside the joint can get inflamed just like the surrounding tissues. 

If associated with capsulitis, synovitis may be treated in similar ways as describe above.  But if a result of infection, arthritis or other causes, synovitis may need very different treatment than would capsulitis.  Such treatment may even include antibiotics and surgical repair.  

There are a lot more tendons in your body than you may think.  Consider just the toe, for an example.  In most cases, there are two tendons to pull the toe up, two tendons to pull the toe down, not to mention the dorsal interosseous muscles and tendons, the plantar interosseous muscles and tendons and lumbricale tendons, primarily to stabilize the toe and help make the pull of the major tendons more efficient.  

Symptoms of tendinitis include pain with pressure, joint motion (particularly against resistance), and if chronic enough, misaligned toes may develop.   

Conservative treatment includes rest, ice, immobilization, physiotherapy, and massage.  Surgical intervention is rare, unless the tendon has become ruptured, or degenerative, (a condition called tendinosis), in which case, it is better to fix the problem earlier rather than later. 

This website has a web page that covers tendinitis in greater detail.

Tenosynovitis is an inflammation of the fluid that keeps the tendons lubricated inside their sheath.  If allowed to progress, this condition may cause erosion or scarring of the tendon itself. 

Symptoms of tenosynovitis would be similar to tendinitis--pain with moving the foot--only more severe.

Conservative measures--rest, ice, anti-inflammatories, physiotherapy, e.g.--may all be useful, though surgery is often necessary in severe cases where the tendon has degenerated.