If your hosting package allows more than 1 domain then you can host as many sites on the same webspace as the package allows, a basic webhosting package usually allows 1 domain but if you have a business package or a reseller package then I am sure that you can host more than 1 domain. As NickStagg said Sub-domains are an extension a domain you have hosted, if all you have is a basic one domain package you can create a seperate cpanel for a sub-domain and host a different site there , you can then forward/frame a new domain to that sub-domain if if you want that site to have its own url., hope this helps.