The tree is a crabapple and has a case of either cedar apple rust or apple scab - probably apple scab. This is just what many varieties of crabapples do - get one of three diseases (the other is fireblight) and then (often) defoliate.
You can probably tell that crabapples are not my favorite type of ornamental.
Many newer varieties are resistant/nearly immune to these diseases, but for old varieties your only hope is a regular spraying regimen with a fungicide recommended for these diseases. You'll also need to rake up ALL of the leaves that fall and dispose of them through your city/town's yard waste program, if you have one. Do NOT COMPOST them.