Doesn't everyone want to know?
Here is the only information that is, I believe, official:
"Quarians have an endoskeleton, lips, teeth, and two eyes with eyelids and tear-ducts." (Ascension p. 235)
In all seriousness, I can't say much more then that besides the patently obvious. Quarians are humanoid and it is likely that their facial structure is similar to humans based on the above statement. They also seem to be similar to humans in the fact that they are mammalian and warm-blooded.
Some major differences appear to be the number of digits on the hands and feet. The hands have only two fingers with an opposable thumb. Their leg structure is also a variation on the human model. Their shins curve to the back of the subject, a curve that continues into the foot so that the balance point at the base of the toes is directly under the knee and consequently the rest of the body. This results in the subject's weight mainly being near the front of the two-toed foot. This suggests that the evolutionary ancestor of the Quarian was also nomadic, as this system is quite efficient for running and reduces the energy required to maintain a walking stride in comparison to human legs. Though this structure is referred to as "chicken feet" by some, it is important to note that the knee of the Quarian bends in the same manner as a human's.
Other known minor variations include an exaggerated profile. This is to say that the Quarian appear to be broader at the hip and shoulder and thinner at the waist and neck then the human average. Certain elements of this are more obvious depending on the gender of the individual being observed. It is unknown if they have any form of body hair.
The Geth share a similar structure outside of the infamous "flashlight head."