Guild Wars 1 has Attributes, alterable point scores in four or five disciplines for each class, which govern how powerful the skills of that discipline are, and also which carry some innate passive effects as well. For the Memser, these would be Fast Casting, Domination, Inspiration and Illusion. One of these would be the Primary Attribute (Fast Casting for the Mesmer), and unique to that class. It was this, and the armour piece stats, which would make a Mesmer/Ranger subtly different to a Ranger/Mesmer, who would get Expertise instead.
Guild Wars 2 has gone with a more conventional Talent Tree style of customisation, often using the same names, but something of the spirit of the Primary Attribute lives on in the Profession Mechanic. This is different for each class and usually involves the F1-F4 keys, and the UI area just above the Weapon Slots. For the Mesmer, this is Shattering.