Any construction project, including renovations and maintenance, will involve many different trades. You need an electrician, a plumber, a roofer, a painter. Who is going to manage all these people?
Typically, the owner of the property under construction hires a general contractor. This individual or company is responsible for hiring, coordinating, and supervising sub-contractors (also called specialty trade contractors and sub-trade contractors). In other words, the general contractor sub-contracts out work to other tradespeople.
Sub-contractors perform specific work on one aspect of the construction project, such as the electrical wiring or the painting. A sub-trade is thus the trade a particular sub-contractor specializes in.
Note that the terms “sub-trade” and “sub-contractor” are more relevant in construction projects because of the involvement of a general contractor. Without a general contractor, there is no need for the term sub-contractor.
For example, an electrician working on a construction project is considered a sub-contractor, but the same person working outside of a construction project would not be called a sub-contractor, just an electrician.
It follows then that any construction-related trade could be a sub-trade. These activities begin from the initial preparation of the property for construction to foundational work to the actual building.
References:
http://www.canadabusiness.ca/eng/page/4875/
https://www.ic.gc.ca/app/scr/sbms/sbb/cis/definition.html?code=238&lang=eng