that depends on what the datacentre is built for. if it's a colocation DC, then every customer uses his servers for his own needs, if it belongs to a company, then there is more order in the chaos.
there are a few major types of virtualization - desktop, software, server, storage and v-cloud (a very generalized concept that includes all of the aforementioned types).
desktop virtualization comes in two main flavours - desktop VMs (vmware vdi, red hat/qumranet solidICE, XENdesktop) and terminal servers (MSTS, Citrix, LTSP)
software virtualization provides provisioning of software through different means, starting from terminal sessions that provide only one program and close after you exit that program, and down to things like http://www.installfree.com
server virtualization is for virtualizing server grade solutions. most commonly used systems are vmware esx, XEN, Parrallels, KVM/QEMU, VirtualIron etc...
storage virtualization is basically what you get in a modern SAN - NetApp, EMC, Hitachi
and as for v-clouds - just read the wiki article, the concept is too wide for a brief explanation