The guardian focuses primarily on increasing their physical and magical damage mitigation while keeping their group members alive. Guardians are more heavily armored than any other tank class, specializing in the use of their shield. They have very low damage capabilities, but like the berserker, they can use their tactics to buff the offensive and defensive abilities of their group. In return, they gain skills to build tons of threat on their targets and to take more damage than any other class in the game, making them an indispensable character for small groups and raiding. Their low damage makes soloing very low risk, but time consuming, so they generally spend most of their time grouped unless they have to solo.
When tanking, it's generally best to stick to your defensive stance unless you're having a great deal of trouble maintaining threat, in which case you should switch to offensive. Guardians can increase their own health totals and have a couple of buffs that increases their mitigation in return for either slowing them down or completely rooting them to the ground. This should be used in situations in which heavy mitigation is called for and you don't need to move around too much to avoid wandering monsters or environmental effects. Guardians have more taunts available than any other class in the game, particularly focused on single target threat generation, but they also have several area of effect threat skills.
Guardians also have a great array of intervention skills that allows them to take attacks in place of ones that would land on their allies, helping them to keep their group alive much longer than they would be able to otherwise. A skilled guardian needs to be quick to react if they lose the attention of a monster, as failure can easily lead to a full party wipe.
A skilled guardian needs to build threat on their target as fast as possible to prevent it from running to one of their party members due to a stray heal or buff. Make sure that your party knows that, and holds still on damage or any other abilities before you have the monster solidly. As you improve, you should become more accustomed to the pace at which you need to keep monsters coming at the group. If you're in a group with another tank class, it's generally the best practice to assign the guardian as the main tank, as they have very limited utility if they're not actively tanking something.