English Ceilidh (with Ceilidh being pronounced 'kaylee') is a more riotous and eclectic variation on English Folk, Barn or Country dancing. Simple Folk Dances done with energy to music deriving from Irish to Caribbean.
It's what you get when you take the gentler English Country or Folk dancing and add excitement. The form of the dances is basically simple. The quality of the dance is very much dependent on the band plus how well the band, caller and dancers get on with each other.
The new wave of English Ceilidh bands use a mix of traditional and recent tunes and variety of styles; there's as much Rock, Jazz and Roots as there is Folk.
The Irish 'Ceili' describes more of a party with dancing; 'Ceilidh' is the Scottish spelling (see Wikipedia for the accents). The meaning of the word has changed over the years, in the Gaelic tradition it originally meant more a gathering with 'stories and tales, poems and ballads' than dance. 'English Ceilidh' has its roots with these dances and events but has developed a style of its own....
There are several collections of dances on the Web:
The Round has a whole list of descriptions about the whole list of moves, this covers far more than you would meet in an English Ceilidh but you might need more of them in a dance event. Webfeet also has a small but growing collection of annotated dances
There is a English Ceilidh mailing list running which has discussions, announcements, information etc. You can join and just listen, or join and get involved. A proportion of the list members have put in contact details into a members list.
... In England, a 'barn dance' suggests either a community organised event or an English Folk dance with a substantial addition of square dances. You don't see so many square dances at an English Ceilidh but you are likely to have Polkas, Schottisches and Waltzes included instead.
A common description is that Ceilidh tends to involve 'stepping' and folk or barn dances not - although it is seldom said what 'stepping' is. This is more an observation rather than a definition; but in Barn dances, Square dances, Contra's, English Country Dance etc the dancers often walk the dances. In ceilidh, whether it is the traditional form from the North East of England or the more cosmopolitan Southern English Ceilidh, you seldom do a plain walk.
A somewhat irreverent rule of thumb for a band is that if it includes an accordion and is sitting down, then you are likely to get a barn dance, if it includes a saxophone or trumpet and they are standing up, then you will get an English Ceilidh. Watch out though - this rule of thumb only works south of the border as Scottish Ceilidhs generally come with Accordions. An additional complication is that a Ceilidh in its original meaning is more of a party or gathering with dance only a being a part, dance events seem to be 'Ceilidh Dances' in Scotland....
It is not only English Ceilidh which has trouble deciding on a name, on the other side of the pond there is also debate about their Contra, Barn or Country dances..
... well apparently Contra dancers don't skip but use the energy left over in twirling. Contra is generally a smoother, faster dance with smoother music, reels rather than hornpipes. There is a range of energies in both English folk dance and American Contra dance, however Ceilidh being at the high energy end of the English folk dance spectrum it is more consistantly exhausting. If you don't recognise the term 'Contra dancing' then there's less to worry about but look up Gary Shapiro's description and Doug Plummer's remarkable photos.
Bob Archer has also put together a detailed comparison of the English and US dance scenes. For something a little deeper, Neffa is building a collection of essays which shows that the same questions can pop up on both sides of the Atlantic.
If there is a rule of thumb for bands is there a similar rule of thumb the dancers? If there is, then it is on the feet. Watch the floor and if you see mainly trainers then you are at an English Ceilidh, 'Sensible shoes' and you at a Barn or more traditional Folk Dance. Analysis suggests that the energetic, off the ground Ceilidh style requires flexibility and grip (somewhat like squash or badminton). In the 'English Dance' style however, often walked rather than danced and where you are looking for economy on effort in the faster dances, grip can be an embarassment - you need to slide. A final rule of thumb here is that if anyone is wearing high heels, find a pub and enjoy a drink instead. As in any Rules of Thumb - your mileage may vary.
Ceilidh and Folk/Barn dance events also differ in the relative importance of the caller. A folk, barn or English Country dance stands or falls on the quality of the caller, it is he or she who determines the tone of the evening. If the band has an off-day but the caller is good then the evening is good. With English Ceilidh it is the band who sets the tone.
This difference in emphasis can be seen in the advertising - English Ceilidh events advertise the band with the caller, Folk and Country dances normally give the caller and then the band. (In fact Folk or Barn dances can be run on a small scale with a caller and recorded music. English Ceilidh events are always with live bands).
For some context and history:
It is worth having a look at Hugh Stewart's Caller's Check List [pdf] as well. This is an invaluable list which describes the sort of things which come as part and parcel of calling for a ceilidh/dance. On the same site there is a more general resources page with a lot of good information including comments about insurance.