THE PROBLEM:
Two or more sentences are joined together by a comma or written as one sentence.

Examples

The skiing industry in British Columbia is booming, both skiers and snowboarders are flocking to their local hills to enjoy the rush of cruising the slopes.
or
The skiing industry in British Columbia is booming both skiers and snowboarders flock to their local hills to enjoy the rush of cruising the slopes.

HOW TO FIX IT:

Separate the run-on into two sentences.
The skiing industry in British Columbia is booming. Both skiers and snowboarders are flocking to their local hills to enjoy the rush of cruising the slopes.
Connect the sentences with a comma and a co-ordinating conjunction.
The skiing industry in British Columbia is booming, for both skiers and snowboarders are flocking to their local hills to enjoy the rush of cruising the slopes.
Connect the sentences with a semicolon.
The skiing industry in British Columbia is booming; both skiers and snowboarders are flocking to their local hills to enjoy the rush of cruising the slopes.
Add a subordinating conjunction to one of the sentences and use a comma before the other sentence. (In the following example, the sentences have been inverted.)
Because both skiers and snowboarders are flocking to their local hills to enjoy the rush of cruising the slopes, the skiing industry in British Columbia is booming.