Crown land is land that is owned by the federal government - it was never sold off/divided up/ land granted etc. It's land that hasn't been turned in to national or provincial parks.
The individual provinces manage the land on behalf of the feds so each province makes their own rules about usage. Because each province manages their crown land, some provinces make it easier to camp on than others.
Ontario, for example, makes it somewhat difficult to car camp because it's all in northern ontario and a lot of the land is forestry managed. Once each forestry company is finished logging, they have to decommission the roads to release themselves of liability issues. Decommissioning the roads usually means tearing up the entrance access to it so you can't drive on them. I mostly found car camping on crown land at old boat launch areas that the MNR had put in decades ago. Residents are welcome to stay for free for up to 14 days before needing to move, non-residents can get a permit for camping that's really cheap. The ones you can camp at now are all in unorganized townships because the MNR downstreamed as many as they could to the municipal level who promptly put up all kinds of signs...sigh!
BC, on the other hand, encourages use of the crown land and has developed some 1,300 smaller camping areas that are mostly now under the guidance/jurisdiction of the Ministry of Tourism. Some have a very small charge to cover the cost of maintenance of vault toilets, campsites, garbage removal etc. There is also undeveloped crown land. I'm about to start more exploration of the campsites they've developed.
To find crown land in each province for camping, simply google 'crown land camping in xxx' and start following the leads. Most provinces have crown land mapping that is available on line these days. Here's the one for BC -
http://www2.gov.bc.ca/gov/content/data/geographic-data-services
The other way is to get to know the locals and ask them.
Crown land is government land. 
