IGMP

[#1]

Our BT Voyager 2110 router has an option to enable Internet Group Management Protocol.  By default this is disabled, which surprises me, which probably means I haven't understood what I have read about IGMP.  In addition to a desktop running Ubuntu on a wired connection, a desktop and laptop running XP wirelessly, we now have an Internet TV. All these devices receive Youtube and BBC iPlayer, though the iPlayer sometimes freezes for a short while on the TV.  We do not game online or download other media.
We are considering installing software to enable viewing on the TV of .MOV and .JPG files stored on the computers; the software suppliers suggest enabling multicasting.  Can someone please explain, preferably in not-too-technical language, whether IGMP is, in effect, the same as multicasting, why the IGMP is disabled by default, and what would be the effect of enabling it? 

[#2]

IGMP is an integral part of the IP multicast specification.  It's primary use is for multicasts between disparate networks, e.g. over the internet,  and is intended to control what is effectively a broadcast  to be directed towards those that are interested in receiving the it.
IGMP is probably turned off by default due to some security concerns and the limited use of multicast over the internet  (VOD is unicast).
Within a home network with a single router, IGMP would be redundant as all traffic goes through the router anyway.

[#3]

Many thanks, that makes sense.
Assuming that by  VOD you mean Video on Demand, we don't actually want that, though many users of the software (Twonky) probably would. Is it possible that although unicast, the large packets of data used by VOD would travel better with multicast? Otherwise, have you any idea why Twonky recommends enabling it?

[#4]

I did mean Video on Demand, yes    (4OD, iPlayer, etc). 
If streaming to a single client, there is no significant difference between unicast or multicast.
For multiple devices,  unicast allows each of them to work independently,  multicast allows more than one device to be in sync when processing the same multicast stream.
Switching between unicast and multicast, while theoretically possible, is extremely unlikely as it would provide a massive synchronisation headache.
Some live video streams on the net may possibly be multicast, but 'live pause' and the like would have to be implemented on the client side (like Sky+ and PVRs).
By the look of things,  Twonky uses multicast as it is working according to the DLNA spec and should be compatible with other devices that support DLNA. 
A quick check indicates it's primarily used for finding compatible devices on the local network, but may well be used for sending the streams too.
So using multicast is fine and preferable to allow all compatible devices within your network to work.
IGMP is still only relevant if you want to route multicast between different networks.

[#5]

Thank you for your patience and well written reply - even I understand it.!