An ether linkage is an oxygen atom which serves as an intermediary bond between two carbon atoms which are themselves bonded only to other carbon atoms and hydrogens. A glycosidic link is the same thing, but only when one of the molecules is a sugar, so in polysaccharides and DNA.
You don't really need to know the principles of thermal crack, I believe, but you need to be aware of what the functions of cracking are. Just know that thermal cracking involves applying great heat and pressure to a hydrocarbon in order to form a smaller one; often in the VCE this involves breaking an alkane into a smaller alkane as well as an alkene (unsaturated).
There's also catalytic cracking, which you don't need to know, but make sure you don't get confused if you hear or read this term anywhere else! That involves using a catalyst to crack the alkane.