1. Outskirts means the outer parts of an area. It is normally used to refer to the outlying areas of a city, but it could have other uses as well. Outskirts is the normal usage. I checked and there is some mixed information about whether outskirt can be used. I would stick to outskirts. Note that the "s" at the end of outskirts does not refer to a plural form, it is just a noun representing a particular location. For example - "We lived in the outskirts of Sydney", meaning we lived on the outlying or outer border area of Sydney. 2. Suburbs refer to the residential areas outside a city centre. There is a slight difference to outskirts. In point 1 above, it means on the outer edges, whereas in point 2 it means not in the centre, hence a subtle distinction. For example, "We lived in the suburbs of Sydney", means we lived outside the central city area. It may not necessary mean that we lived on the outer edge of the city as in the first case. This, as well as the fact that outskirts can be used in other circumstances, for example, you could say "The explosion was heard even at the outskirts of the firecracker factory", meaning at the outer edge of the factory area. The above is my understanding, I'm not sure if the two terms are interpreted slightly differently in the US.

Suburbs are a part of cities. Outskirts can be related to anything spatial. You can say that 'the suburbs on the outskirts of the city' which refers to the suburbs on the very edge of city - far out. You can have suburbs that are not so far out from the city center, but you would not say they were on the outskirts. Outskirts is reserved for something at the edge. Outskirts is not always plural, however you are far more likely to see it in the plural. 'The cabin was situated on the outskirt of the plain' for example. However that sentence is also correct with the plural, outskirts. It is a bit of an old word. I hope that has helped. Alexis

The answers below are pretty good, but outskirts usually means the area immediately outside of the suburb you are talking about. Most often outskirts will be plural because many areas are outside of the current area, there's the suburb above, the suburb below, etc. You can use outskirt as singular however the use would be less common, for example if you draw a line around the suburb could point to the line and say "this is where the outskirt begins" or if you were standing on the border between suburbs you could say "I am standing on the outskirt"