Side note:  Spell check knew "glossolalia", but not "xenoglossy".

Both  can be referred to as "speaking in tongues", but they are different and (as one website put it) should be distinguished from each other as often as possible.

Definitions:

Glossolalia. From (from Greek γλωσσολαλιά and that from γλῶσσα - glossa "tongue, language" and λαλεῖν (lalein) "to talk") - this is copied and pasted so if it's not exactly correct, hopefully it's good enough for a lay person.
Trying to find an "official" definition that clearly defines glossolalia as different than xenoglossy and isn't biased in terms of Continuationist vs. Cessationist has turned out to be an interesting side trip.  It was easier to get the definition from a secular dictionary.

The ability or phenomenon to utter words or sounds of a language unknown to the speaker, especially as an expression of religious ecstasy. Also called glossolalia, speaking in tongues.

The important key words are "words or sounds".

A less generous defintion (both from answers.com):

Fabricated and nonmeaningful speech, especially such speech associated with a trance state or certain schizophrenic syndromes.

Xenoglossy.  (from Greek ξενογλωσσία - xenoglossia, from ξένος - xenos, "foreign" + γλώσσα - glossa, "tongue, language").

This was an easier defintion (from worldwidewords.com):

The ability to speak a language without having learned it.

The important key word:  language.

Compare:

• both glossolalia and xenoglossy use "sounds" or "words" that do not belong to a language that the speaker knows.
• Both can be associated with religious activity.

and contrast:

• glossolalia belongs to no known language.  Pentecostals call this "tongues of angels" and it may be referred to as a "private prayer language" that nobody (except presumably God) can understand.
• xenoglossy can be identified as a real language and can be understood by a person who speaks that language.  In some cases it is claimed that a 2-way conversation has taken place between a native speaker and the "tongues-speaker" who has never been exposed to that language.

In short:  real language vs. not real language.

Other handy definitions:

• Cessationism:  the view that the charismatic (or prophetic) gifts of the Holy Spirit, such as tongues, prophecy, and healing, were only given to the new church for a period of time and ceased either at the end of the apostolic era or shortly after.
• Charismatic: the general term used to describe Christians who believe that these same gifts are available to Christians today (see "Continuationism").  Closely related to Continuationism, and I'm not sure of the difference, other than continuationism is the belief and charismatic is the movement.  Also see "Pentecostal" - although, you can be charismatic without being Pentecostal.    Mark Driscoll refers to his church as being "charismatic with a seatbelt", which I think is a way of saying, "we're charismatic but not Pentecostal."
• Continuationism: the view that these same gifts continue to this present age and are available for all Christians alive today.
• Pentecostal: the belief that a "born again" Christian can (and should) receive a subsequent experience of a "baptism of the Holy Spirit", the initial evidence of which is speaking in tongues as the Sprit gives utterance.  Some Pentecostals believe and teach that a person who has not spoken in tongues (received the baptism of the Holy Spirit) is not saved.  All Pentecostals believe that the "gift" of tongues is THE gift that proves the baptism of the Holy Spirit (Acts 2).  The other gifts were given later and a believer does not have tongues or another gift...it is always tongues plus the rest of the gifts listed later.

Here's the problem:  Christians are not the only group of people who experience glossolalia and xenoglossy.