It's pretty much anything you can imagine. There are two general categories of assets. These are assets that are used in the commission of an offence. These are offence-related properties such as a vehicle used to smuggle, a property used for a clandestine lab, etc., and then there are the proceeds of crime themselves: the cash, the fancy cars, the luxury properties that folks would buy. They also include things such as businesses that can be used to launder money.
Prior to conviction, these assets, although seized, remain the property of the accused, so they need to be maintained. A business may need to continue to be run or a luxury vehicle may need to be preserved in the state in which it was seized. Even a residence may continue to be occupied by the accused while the process unfolds, and that can take years.