C-Reactive Protein (CRP) is an 'acute phase protein' and is produced in the body in response to inflammation.

Although we know a little of what CRP does in the body, there are probably quite a lot of influences that are yet to be discovered.
CRP rises quickly in the presence of inflammation or infection so is often used by doctors to see how likely it is that you might have a serious infection.  It can also be used to help to distinguish betwween inflammatory and non-inflammatory diseases.

In general terms, CRP most often lets the doctor know that something is going on, but on its own gives little more information.  It is really important to use this test in conjunction with the symptoms you have to make a diagnosis.  If your doctor is having difficulty working out why you have a raised CRP please have a think about whether or not you have revealed everything you have noticed as even small symptoms can make a difference - a large boil might be the reason why you have a mildly raised CRP and if you are asking about joint aches it might slip your mind.

Here are some of the reasons we might check your CRP:

Polymyalgia Rheumatica.

There are many conditions which cause all-over aches and pains.  A raised CRP (well over 5 and usually over 15) suggests that PMR may be the cause and a course of steroids may help.  A low CRP does not exclude the diagnosis but does make it a lot less likely.

Viral or Bacterial infection.

If you have some signs of bacterial infection (perhaps a chesty cough and some crackles in your chest) but some other indications suggest a viral cause (no fever, runny nose) then a CRP might be useful to decide whether antibiotics will do more good than harm - in general a level under 10 suggests antibiotics can be avoided.  At higher levels, antibiotics become progressively more likely to be useful.
In abdominal pain, low levels suggest a non-serious condition but a higher level might be more consistent with appendicitis.

Inflammatory Arthritis.

A high CRP suggests joint pain may be more likely to be due to inflammatory arthritis (such as gout or rheumatoid disease) than due to degenerative diseses like osteoarthritis.

Lethargy.

Inflammation is a rare cause of lethargy so CRP is often part of a panel used to ensure this is not the case. 

Non-Specific Symptoms.

Many common symptoms, being common, will occur together or in what appears to be a relevant sequence.  Although these rarely represent serious disease and often occur together by chance alone, CRP levels can sometimes be useful to help to ensure that nothing serious is being missed.  In such circumstances a normal CRP (less than 5) is very reassuring but higher levels do not often mean there is anything serious either.  The test is just part of a bigger picture.

High-Sensitivity CRP:

This test is used only for assessment of heart disease risk.  The higher the result, the more likely you are to develop heart disease - those with the highest levels have four times the heart disease risk of those who have the lowest levels.  The usual range is from 2.0 to 4.0

There are many other reasons why CRP might be raised.  The key to interpretation is ALWAYS the patient's symptoms.