^^eagle23 nailed it dead on^^
My Dad has a factory repair manual for 1970 Ford Trucks that explains it best. There are two type of tire imbalance; tramp and wobble. Tramp is the action of a tire hopping up and down like a basketball. Wobble is obviously movement side to side.
Static balancing gets rid of tramp, but doesn't get rid of wobble. Static balancing is done with a bubble level stand or by deprogramming a computerized spin balancer so that it does a simpler job.
Dynamic balancing gets rid of both tramp and wobble, and must be done by a computerized balancer. The first dynamic balancing machine was invented shortly after WWII.
If a tire isn't too far out of balance, static imbalance can usually be felt under 50 mph, but dynamic imbalance isn't felt until over 50 mph.
The next step is road force balancing, which takes into account the variations in flexibility found throughout the sidewalls of a tire.
Around here, we are the only tire & alignment shop that regularly dynamic balances. Most other shops have more expensive balancers, but just do a static balance out of laziness. I believe a huge tire shop that was built in our town with ex-hacienda money had a Hunter 9700 road force machine before they went out of business, but if they did I am certain none of their employees had a clue how to use it.
In conclusion, you want the best, most precise balance possible, a good balance equals less money spent on ball joints, tie rods, bushings, &c. Road force if possible, dynamic if not. The only time a static balance is viable is if your tires are uneven (mud or Chinese tires) or if you have Chinese rims, or deep offset rims that don't have lips for clip-ons and have the spokes set too far inwards to effectively use adhesive counterweights.