Syndactyly is a name that means webbed fingers. Webbed toes is when two or more toes are fused together. In early fetal development, all human's toes and fingers are webbed together. They are separated by apoptosis that occurs by a specific enzyme. Therefore, webbed toes occur when this apoptosis either does not occur or is incomplete.
There are six different types:
Simple: adjacent fingers/toes joined by soft tissue and skin
Complex: bones of adjacent fingers or toes are fused together
Complete: the skin is joined the entire length of the digits
Incomplete: the skin is joined partially up the digit, usually to the first joint
Fenestrated: the skin is joined for most of the digit, but there is a gap in the middle of the joined skin
Polysyndactly: there is an extra digit webbed to an adjacent digit.
The cause of this is still unknown, but women smoking during pregnancy or a women's nutritional intake during early gestation can contribute. It is a side effect to over 100 different diseases, and it does occur in families.