Amblyopia, better known as lazy eye, is where is one eye drifts while the other remains in line. ("Lazy" does not refer to the misalignment of the eye. Instead, it refers to the vision impairment that is caused by eyes misaligned.) When someone has a lazy eye it means that their brain is actively ignoring the weaker eye, and chooses to only receive images from the stronger eye.
The key difference between lazy eye and crossed eyes (strabismus) is that lazy does not refer to the misalignment of the eye. Instead, it refers to the vision impairment that is caused by eyes misaligned. Where strabismus is caused by weak muscles, amblyopia is caused by one eye being tuned out by the brain. Not everyone with strabismus will experience amblyopia, but it is very common. The brain will begin to favor the stronger eye and ignore the weaker one, leaving that one to drift even further. Another thing that can cause amblyopia is uneven refractive errors in each eye. A child with a refractive error will have perfectly aligned eyes, but if one eye sees better than the other, then the brain will ignore the weaker eye.
Lazy eye is the most common vision condition among children and is very treatable if treated early during childhood. 5% of all children are born with some sort of refractive error in one or both eyes. If one eye cannot form sharp images, the brain will tend to ignore it. Unless this amblyopia is diagnosed at an early age, the full use of the 'lazy eye' can be irretrievably lost. Early diagnosis is most is difficult in relatively mild cases and where only one eye is affected.