Question: I have never really understood the connotation of someone calling their domicile a *condo* over the word *apartment*. I have a vague feeling the former is fancier and more up-scale, but are there any real differences I'm not aware of?

Answer: 
There is a big difference in the USA. In a **condominium**, the ownership of a building is divided into shares and ownership of some of those shares gives the right to live in a particular part of the building. In the event that you want to sell those shares, the other shareholders in the building must approve the transfer, so if they do not like the new purchaser they may block the transaction. Equally, some condominiums restrict letting the property. 

In a Co-op, the **apartment** is owned by the individual and in many, while the management company of the building has the right to refuse transfer to a new purchaser, it instead must buy the property itself. Although usually cheaper, Condos are viewed as more 'snobby' because there is a degree of social acceptance that goes with them.

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**EDIT**: Not sure I agree with all the answers following my first post. 
When I lived in NYC I owned an apartment in a co-op, a high-rise in NY10021. Some of my neighbours were renting, others were owner occupants. All were called apartments and labelled as such on the mail boxes in the lobby. The bank would not lend to me (resident alien) to buy a condo, because of resale/renting issues should something go wrong, which is why I bought (and could only borrow for) a co-op. It might be different outside NYC, I've no property-owning experience there. 
Some people bragged about ‘condos’ and saw a benefit in owning one. I thought this was rubbish, as I considered having to flaunt supposed ‘social acceptance’ as pathetic. I also believed that the big disadvantages inherent in that type of restrictive ownership outweighed the relatively minor advantage of lower acquisition price.

In Europe, a flat is an apartment, either owned or rented, and colloquially means either a student’s rented accommodation, or a second, smaller residence of a temporary nature e.g. ‘I keep a flat in the City for a convenience.’ Sometimes the latter is called a *pied-a-terre*, literally (French) ‘a foot in the ground.’ A flat often is used to describe a unit in a sub-divided property, whereas an apartment usually is in a purpose-built block. The terms 'condo' and 'co-op' are not used for property in Ireland/UK/France. BTW, in Ireland and the UK a *co-op* is where a farmer buys cattle feed ;-)
Condos tend to be in complexes rather than single buildings. Additionally, a condo is purchased and owned, whereas an apartment is rented.

[This Link](http://www.realtor.com/Basics/AllAbout/HomeAffordability/Condo.asp) has some good info about condos.