Updated after BWE#3
When you first start out you don't have this ability. Once it does unlock you can have two groups of weapons setup to use.
As an example lets look at a warrior.
He or she can use the following:
Axe, Mace, Sword (these can be used in the main hand or off hand and duel wielded).
Shield, Warhorn (off-hand only)
Greatsword, Hammer, Longbow, Rifle (Two-handers)
Spear, Harpoon Gun (Underwater combat)
So the warrior runs into something mean somewhere in Tyria. He or she has the Longbow equipped and opens the attack against their foe. This brings the attacker in close to the warrior who then hits the swap weapon button and brings out an Axe and a Mace. The warrior stuns their opponent and then proceeds to finish it off.
Now lets look at the Elementalist. Since they have access to the four elements (Air, Earth, Fire, and Water) they have a unique situation. Each element having to be trained up for what ever weapon you choose to use. (This I found to be the only profession that I felt truly had to grind btw.)
First off you have the weapons they can use.
The Scepter and Dagger for the main hand. (Note: Dagger can be used in the off-hand as well.)
The other off-hand weapon is a Focus Item.
Lastly is the two-handed Staff. Each of these weapons have a certain range. The Staff being long range. The Dagger is medium range, while the Scepter and Focus are close range.
Instead of being able to swap weapons during combat you get the chance to swap out the fifferent elements. Water gives you 2 options to heal if you use the Staff for example. While air give you a lightening whip and a static force shield that hurts a baddy that hits you while it is active. These can be seen while using a dagger and focus.
This is one of the elements that has helped to make this MMORPG different from the others that are out there. The clock is ticking till the release of the game and it is best to seriously start looking into getting it for the head start time which will begin on August 25, 2012. (official launch day is Aug. 28).