Magic weapons are artefacts of bloodshed, pure and simple – they grant the wielder extra abilities or an increased characteristic profile so that they might better slay their enemies.
Unless otherwise stated, a magic weapon is treated as a hand weapon (replacing the model's regular hand weapon), and follows the rules for such. This means that a magic weapon can be used with both an additional hand weapon or shield/buckler following their normal rules. In the case of non-magical additional hand weapons, the +1 Attack does not benefit from any special rules from the magic weapon.
In order for a model to be able to choose a particular piece of weapon (such as great weapon, polearm or two hand weapons) they must be able to pick this as a mundane option or have it as part of their default equipment. Note that magic weapons that count as normal hand weapons may be taken by any model that can choose magic weapons.
What's in a Name?
The magic weapons listed below often have a name that describes them as specifically being a sword or another particular type of weapon. This doesn't mean that the model has to have a sword to use the 'Ogre Blade'. We can simply assume that their axe, hammer or other suitable hand weapon has the same properties and is, for example, an 'Ogre Axe'.
I'm Using This One
A character that has a magic close combat weapon cannot use any other close combat weapons (his magic weapon is their pride and joy, and he's sure as sunrise going to use it). If a character has more than one magic weapon, they must choose which one to use at the start of the combat (unless a rule specifies that both can be used at the same time) – the chosen weapon must be used for the duration of the combat.