Will this lid fit my Crock-Pot?

It is made for 6-quart round Crock-Pot slow cookers. Round and oval lids do not interchange, so confirm your cooker is round and 6 quarts before ordering.

Is this a whole slow cooker or just the lid?

Just the lid. It is the replacement glass lid, part 129962000000, and does not include the heating base or crock.

Does it come with the knob?

The listing describes a genuine replacement lid for part 129962000000. Confirm with the seller whether the knob and handle are included before you buy.

Can I use it on a different brand of slow cooker?

It is built to Crock-Pot's own dimensions for 6-quart round models. Fit on other brands is not guaranteed.

Is a genuine lid better than a generic one?

A genuine Crock-Pot lid is made to the original dimensions, so it seats and seals like the lid that came with the cooker. Generic lids may fit loosely.