(Normal caveats about how you are totally free to run the game however you want and refluff/change appearances) -
The 3e aasimar was simply described as "This being looks like a beautiful human, with calm, serene features and an inner radiance that shines from its face." I don't have the books for Planescape or 5e, but near as I can tell this is still roughly true of the race. Although 5e aasimiar tend to have tattoos or scarification of angelic symbols and, of course, the special form which adds a lot of overtly angelic features.
4e didn't have aasimar per se, they had Deva which are similar but not the same thing.
So basically a pretty human, except when using granted racial powers.