I've heard townhomes and rowhouses used interchangeably. Usually no yard, but perhaps a little bit of fenced-in private space in back.
If it's one house split into 2 separate residences, or 2 houses with a common wall, I think of those as "duplexes".
I've lived in a "foursquare" - it's like 4 rowhomes, but in a square, rather than a row. Ours was something like this:
| | | Each unit had a "front door" on one side (Where the D's are in the pic).
D| | | D The other exterior wall had a door and patio with access to our yard.
|---------------| I think the neighbors in back each had their own, but we shared the "front" yard
D| | | D with the front neighbors.
In New Orleans, some folks have a "double shotgun" - it's a duplex in the shotgun house style. I think of a "single family home" as one that is fully detached from all residences, and typically has at least a token yard, but in my house hunting I've seen plenty of condos (like an apartment, but you own instead of renting) referred to as "single family homes".