This page describes the nature and diagnosis of these ligament injuries.

ACL and CCL are two terms which refer to the same ligament.
'ACL' stands for 'Anterior Cruciate Ligament' 
'CCL' stands for 'Cranial Cruciate Ligament'
These describe in Latin the foremost of the pair of ligaments which cross ('cruciate' means 'crossing') at the stifle.
'Anterior' means 'toward the front'.
'Cranial' means 'toward the head'. 
In a human, the correct term is ACL, 'toward the front'. 
In a dog, CCL is usually used among vets, but obviously either CCL or ACL is anatomically correct since 'toward the front' is also 'toward the head'.  
If you prefer ACL and anyone ever corrects you, just point out that your dog's head is at the front.

The joints involved in these ligament injuries are similar to the knees in people.  In dogs and other quadrupeds these joints are called the 'Stifle'.  Ligaments are bands of slightly-elastic, tough tissue which connect bones to each other and provide stability to joints.  When the ligaments are damaged, the joint loses stability.  The loss of stability can result in damage to other components of the joint as the bones move against them without the restraint the damaged ligaments formerly imposed.    

Injuries to the CCL of the stifle are common in dogs. These injuries may be sudden or may develop slowly.  There may be months of slowly worsening intermittent mild lameness caused by gradual breakdown of the ligament.  Or there may be a sudden injury to the ligament brought on by some stressful event.  When there has been a sudden traumatic event, the injury often also causes other damage in the joint area as well, resulting in swelling and pain.  A dog will react to the pain by holding that leg up and refusing to bear weight on it.  When the instability of the joint has been caused by a slow degradation of the ligament, the symptoms will probably be less dramatic.  Simply a limp that comes & goes, or a hesitation to sit.

Diagnosis -- Ligaments cannot be seen on x-rays. X-rays can be useful to eliminate the possibility that there are other kinds of injury such as a broken bone, but they do not show ligaments. Ligaments could be seen using MRI, but this is of limited usefulness, expensive, and usually not readily available. When the leg is examined by manipulation, damaged ligaments can be inferred by what is called 'drawer movement' at the stifle. It's called 'drawer movement' because the lower leg bone can be moved against the upper leg bone at the joint in a sliding movement like a drawer in a cabinet. This 'drawer movement' is the most reliable indicator that the ligaments are not holding the stifle properly. But it should be remembered that some dogs just naturally have looser stifle joints than others, and all puppies have loose stifle joints which may seem excessively loose, possibly up until as old as age 18 months. It does sometimes happen that 'drawer movement' is not present (or not clearly present) but a vet will diagnose the problem as a ligament injury anyway because ligament injuries are very common and he sees no other cause for the dog's symptoms.

Misdiagnosis-- Misdiagnosis is common. Dogs with a muscle injury or some other reason for their symptoms may be misdiagnosed as having ligament injuries and their people told ligament surgery is necessary. Dogs with tick-borne diseases may limp and be misdiagnosed as having ligament injuries. Paw injuries, bone cancers, and a host of other actual reasons for the dogs' symptoms have been overlooked and the dogs subjected to inappropriate surgery for a ligament injury they didn't have. Vets prefer to present themselves as confident and assured to their clients. They may claim to be quite certain about the exact nature of the injury even when their diagnosis is really guesswork. The possibility of misdiagnosis should be kept in mind when considering treatment options.

A Too-Rough Exam Can Do Further Damage -- Some vets are overly rough in their manipulation when examining the leg.  I hear from people who watched a vet tugging and twisting their dogs' legs and then had the dogs come home limping much worse than before and in increased pain because of the roughness of the vets' examination.  And I hear from people who left their dogs alone at the vets' for exams and when the dog came home his condition was much worse. Never let your dog be examined without being present yourself to stop the vet if it looks like he's being too rough with the dog's leg.  There is no need for that degree of pressure and twisting to be used in the exam. Don't hesitate to stop a vet who is being too rough.  Too much pressure can crush the vulnerable meniscus and damage other joint components in an already unstable stifle.  If drawer movement cannot be felt with simple gentle manipulation, don't let a vet risk damaging the joint further in an excessively forceful examination.
        Sometimes a vet wants to sedate a dog to manipulate the injured leg. The vet's reasoning is that the conscious dog may be tensing the leg muscles and preventing drawer movement from being apparent. When sedated the dog will not be able to tense the muscles, so the vet will be able to more easily elicit 'drawer' movement if the stifle is unstable.
---- I would not allow a sedated exam on my dog. 
Here's why:
-- An unconscious dog will be even more at risk of being further injured by manipulation of the joint.
-- Even if the vet uses only gentle pressure in checking for 'drawer' movement, there is still risk involved in any use of anesthesia.
-- A sedated exam may yield a better understanding of the exact nature of the injury, but what is to be gained in terms of improved treatment?  Once it has been determined that there are no broken bones, I would choose careful restriction of activity as the appropriate next step whether or not 'drawer' movement could be elicited.*  It makes good sense to avoid diagnostic procedures which have risk in themselves but would not alter treatment decisions.
Many different kinds of injuries respond well to restricting activity and giving the body time to heal & recover. If careful activity restriction does not result in improvement, then a sedated manipulation could be appropriate at a later date in order to determine the exact nature of the problem. 
--- Are you considering another exam of your dog who is improving because you want to get a vet's opinion of the recovering joint?
More manipulation of the joint is NOT a good idea.  Unless there are special circumstances I would not allow any more exams. Manipulation of the joint trying to elicit 'Drawer' movement serves no good purpose after the original diagnosis, and there is considerable risk to an improving dog that the vet will damage the vulnerable fibrous scar tissue that is developing to stabilize the joint.  If Fido is improving, don't let anyone manipulate the injured joint. I get emails regularly from people whose dogs were improving nicely until a vet manipulated the recovering joint causing damage and setting back the dog's recovery or worse. 
---- What if I took the cast off your recently broken leg and said "Now I'll press hard on that leg from the angle where it will be weakest.  That will tell us how much stress it can bear!"  I hope you would have the good sense not to allow that test of the bone's recovery.  Be equally wise on your dog's behalf and avoid joint manipulations if improvement is progressing.
Restoring stability to the stifle joint is the goal of both conventional surgery and of non-surgical treatment.
TPLO, TTO and TTA surgeries use a different strategy.  These bone-altering surgeries change the geometry of the stifle rather than restore stability to the joint. 
If you are considering TPLO, TTO, or TTA, please read the 'TPLO/ TTO/ TTA page here at this website.