Provincially-maintained highways in Ontario are numbered in several different series.
2 to (presumably) 399: regular provincial highways.
Originally these were set up to be a coherent network of routes linking the main towns and cities (see rant below). They may include some partially-limited-access roads or limited-access freeways, such as Highway 115 east of Toronto.
*400 to (presumably) 499: limited-access freeways. *
These are often referred to as simply ‘the 400’, the 401’, etc.
They are often numbered similarly to previously-existing highways nearby: for instance, Highway 407, the toll freeway that runs north of Toronto, parallels the older Highway 7.
This set also includes the Queen Elizabeth Way (universally referred to as ‘the QEW’) from Toronto to the US border at Fort Erie. The first version of the QEW was finished in 1939, just in time to be pounded to pieces by the traffic of the Second World War. It was later rebuilt in the fifties and sixties as a full freeway, but because of its historical nature, it retained the QEW designation and never acquired a 400-series number.
I believe that the Ministry of Transportation refers to the Queen Elizabeth Way internally as Highway 1, but, if true, this naming never escaped the Ministry to the wider public.
500 to 599: secondary highways.
Many of these were lost to the Great Renumbering (see below).
600-699: tertiary highways.
I believe there are only a few of these in the far north.
There are also numbered routes belonging to individual counties and regional municipalities.
Incidentally, Ontarians never refer to a highway as ‘route 401’ or whatever; this usage of the word ‘route’ is a dead giveaway that you are from out-of-province, and probably from the States.
Yes, Highway 403 does exist in two parts. It branches off the 401 near Woodstock, goes through Brantford to Hamilton, and joins the QEW in Burlington. Further east, it leaves the QEW in Oakville, and goes northeast through Mississauga to end at the 401.
If you look at a map, you will see that the two segments of the 403 are connected by a segment of Highway 407, the toll road. IMHO, this segment of the 407 should have been numbered 403, to make one logical whole of the route. But road numbering in Ontario has ceased to be logical…
<rant>
The Great Renumbering
The provincial government led by Premier Mike Harris decided to shave its costs by ‘giving’ many stretches of highway to municipalities. It was deemed that the newly-transferred roads could no longer carry provincial highway numbers, so we had to renumber them and change all the signs and maps. Continuous routes that once logically carried single numbers from town to town now exist under a welter of different names. This unneccesary renumbering has made navigating in the province significantly more difficult for visitors.
</rant>