Typhoon's in Hong Kong are fairly common. They can range in force from really windy, all the way up to throwing trees and buildings around windy. The bad news is when there's a serious Typhoon, everything in the city is going to close down. If you happen to visiting, you might have a day where you're stuck in your hotel, or at the very least unable to visit some of the tourist sites. The good news is, you're not likely going to get caught in a typhoon accidentally. One thing can be said about the typhoons in Hong Kong is that they're very prepared for them.
First of all the local media and hotels are always keeping up with typhoon activity. If you're staying at a hotel and a typhoon is approaching, it's almost guaranteed that they'll have some kind of notice. If you happen to be staying somewhere that doesn't tell you, don't worry, almost every tourist attraction has signs up indicating if a storm in on the horizon.
What you should know about these storms is that it can be sunny one minute, and then suddenly the storm will hit. Don't think that because it's sunny outside that the storm might not actually be coming, so don't ignore warnings. The good thing is that many buildings in Hong Kong are designed to take typhoon abuse, so if you stay indoors, and don't do anything stupid, you should be fine.