Technical refers to the need for skill in handling a particular type of trail feature. When someone says a trail was very technical, they mean that the trail had lots of stuff that challenged their skills. Maybe it had some big log piles to get over, or had very twisty singletrack with trees that grab your handlebars, or big rocks to go over or round, or some steep drops with a turn at the bottom that people often wipe out on.
Basically, if the trail is likely to draw blood, then it's a technical trail. If you do a lot of technical riding, you ride trails that you have a good chance of falling on.
As for the semi-slicks, I use them because they came on my bike and I haven't changed them out yet. After this season, I've decided that I don't like them when the ground is wet. In the dry months during a Chicago summer, they're fine because they give you a lot of contact with the trail. In the spring when it's wet often, I slip around a lot. I've tried to go over log piles and had the back tire just spin on the log because it didn't get traction. Side note: you can get over stuff better in the wet by pushing the bike over rather than pedaling over (once you've gotten the front tire up and over that is).
Next year, I plan to have wide-spaced knobby tires for the spring (which throw mud off better than tight-spaced knobbies), then I'll probably go back to the semi-slicks in the summer. The other thing I like about the semi-slicks is that I can ride to a ride, or just take the mtb on the road and do 30+ miles, without a problem. Knobbies can get real annoying on pavement.