Freemium usually means a free service with an "up sell" to paid premium subscriptions. Examples include Skype, LinkedIn, Flickr, Ancestry.com, Typepad, Dropbox, and many others. Freemium differs from Free Trial in that "a free trial is the fully functional product but only for a time limited or feature limited, supported by advertising. Freemium is always free, but you can purchase additional features or service for a premium price."
Recently at Google IO, a panel of VCs discussed discussed the strategies for Making Freemium work - converting free users to paying customers.
More Info: Freemium in the enterprise