Shared webhosting means having many low-paying clients on one server. Thus, many of the webhosting customers may share an IP address in a virtual setup. Some other webhosting companies are colocated, meaning the boxes belong to the clients and they come in to the datacenter and work on the servers.
The data center leases connectivity and the client is responsible for maintenance of the colocation box (or colo for short). When reporting spam, you most likely will not know the setup of the account in question. However, understanding the different types of webhosting arrangements helps a spam reporter become more thorough in reporting the spam to the appropriate ISPs.