You do not really need this info to get started as it will not make much sense to you untill you get sliding, take a good recomendation and then have some fun learing this later :).
The "a" in a wheel listing indicates its hardness. The a is the measure of durrometer. For street skating, wheels typically fall above 95a. In contrast, longboarding wheels tend to be softer, usually below 85a. The harder "a" wheel, the more it slides. The harder the wheel the higher the roll speed can be if everything else is the same. Harder wheels will ride harsher.
The size, is usually the overal diameter of a wheel. The Bigger the wheel the faster and more grippy it is. This is meassured in mm and if there is only 1 mm dimention this is probally what is being refferenced.
Contact patch is the width of the wheel touching the ground. The wider the contact patch, the more grip and less speed it has.
A lip is the edge of the contact patch on the outsides of the wheels, paralel to the board, think the corner of the wheel. A square lip wheel has a 90 angle between the contact patch and the side of the wheel. A square lip wheel has more grip than a rounded lip wheel. A rounded lip wheel is a wheel that's lip profile is curved.
Cores are the center of the wheel as seen from the side, which hold the outside part on from the outside, and the bearings on the inside. Sometimes they are not visable. But when they are that is called an exposed core. When all the urathane is worn off the outer wheel from sliding the core can be seen from the contact patch and the wheel is used up. If a core is offset that means from the front the urathane outer part of the wheel hangs over one side more than the other. A center set wheel has the core dead center.
Wheel skin is the slick part of some wheels contact patch that is much more grippy than the wheel underneath or a stone ground wheel. It looks reflective almost as feels very smooth to the touch.
Stone ground means the wheel skin is removed somewhat by a machine at the manufacturer, not all freeride wheels have this, but no downhill wheels have this. Having this creates less of a break in period before the full slidey levels are unlocked.
