Dennis,
Tacking and wearing round are both methods of changing course to change the tack the ship is sailing on, but differ in the way they achieve this. Imagine a ship sailing as close to the wind as it can and wishing to sail on the other tack. The shortest distance method is to turn into the wind so that half way through the manoeuvre the wind is blowing straight onto the bow. There is obviously no drive from the sails at this point, and the skipper relies on the momentum of the ship to carry it round until the sails fill on the other tack. This is "tacking".
For all sailing craft, but for square riggers in particular, tacking can fail if the windage (or drag) of the rig stops the ship before it has passed the half way mark of this manoeuvre. The ship now loses control and, if in confined waters, can quickly get into trouble. One way to prevent this is to turn the other way, away from the wind, and perform a large circle manoeuvre. Half way through this manoeuvre the wind is blowing directly onto the stern of the ship and of course the ship retains drive and steerage all the time. The huge disadvantage is that the ship loses a large amount of distance as it has turned away from its intended course. This is "wearing round" and is safer and surer but needs much more time and space to perform.
I hope that has clarified things and not just confused the issue!
Greg