Almost every listing on this site says incall, outcall, or both, and the words are used as though everyone already knows them. Most people work it out from context and get it roughly right. Here is the precise version, along with the things the words imply that nobody spells out.
Incall means she hosts. The location is hers to arrange — an apartment, a condo, sometimes a hotel room she has booked herself — and you travel to it. The address is normally given only once the booking is confirmed, and often only shortly before.
That last part is not evasiveness. An incall address is where somebody lives or works, and a provider who hands it out to everyone who asks has given up the only control she has over who turns up. Expect to be told the neighbourhood early and the exact address late.
Incall is usually the cheaper of the two, because she is not paying for travel or for the time it costs. It is also the more predictable: she knows the space, the parking, the entrance and the neighbours.
Outcall means she travels to you. In practice that is almost always a hotel, occasionally a private residence, and the distinction matters more than people expect — plenty of providers do hotel outcalls only, and say so.
Outcall normally carries a travel fee outside her own area. In the GTA that is usually banded by distance rather than charged flat, because an hour on the 401 is a genuinely different proposition from twenty minutes across downtown. In Vancouver it is quoted by bridge. In Montréal, likewise.
A residence outcall usually involves more screening than a hotel one, for the obvious reason: a hotel has a lobby, cameras and staff, and a house does not.
Most listings state one or both plainly. Where a listing says both, incall is generally her default and outcall the thing she does with more notice. Where it says outcall only, that is frequently a matter of not having central space rather than a preference — it is very common in Vancouver and increasingly so downtown in Toronto.
The filters on each city page let you narrow to one or the other before you start reading.