Whether you get a seatback TV on a long-haul flight depends on the type of plane you are on - but in general, in economy, even on most long-haul flights, you won't get much, other than a flickering overhead screen. If you like to watch films on flight, don't fly on Iberia.
In Iberia Business Plus long-haul you at least get a seatback TV. However, there are two different types of systems. On some of the A340s you'll only get films on a loop, with 12 channels of films, and 14 other channels of sitcoms, documentaries, and music videos. Many of the A340 and all of the A330 aircraft have AVOD, and here it is a full AVOD system, with Video on Demand, that you can play & pause (with the blue button), move forward (red button) back (green) and stop (yellow). There is a selection of games. There is a moving map, and a tail view camera in the A340-600 (on channel 17). You get noise-reducing headphones, with three pins.
Iberia A340 Business class personal TV with tailcam
You can send SMS messages & EMails from your seat using the TV - although it costs €2.10 per 160 characters, so it's not exactly cheap! On the A340-600 (except EC-INO, IOB and IQR) you can plug in a network cable, and get internet access for $5 for 4 hours use.
In Economy on all longhaul flights, there are just overhead screens, so you get the usual two films on a transatlantic flight. There are 12 audio channels, but you will not get any headphones in economy short haul unless you ask. Noone seems to listen to these radio channels any more, but they are quite good, although only one plays current hits, and the rest play obscure Spanish folk music.
Iberia A340 Economy class overhead TVs
Around Europe there is no in flight entertainment: the screens just show the usual map, or stay blank for the duration of the flight. The exception is for the specials. These are specially configured A319 which Iberia use from Madrid to Moscow, Tel Aviv, Cairo, Lagos, and Malabo in Equatorial Guinea. In economy one film is shown on the overhead screens. In Business - called "la Business Club Iberia" - you are given a PSP console preloaded 10 high-definition films, 10 television episodes (one in English), and ten audio channels. Alas, you have to choose between holding it, or putting it on the tray table, and not eating.
Iberia's in flight entertainment content is Ok, but movies on Iberia can be an eclectic bunch. On AVOD there will be at least one first run movie, and some older films from the past couple of years. Much of it is Spanish centric so you may get the chance to see some films you never knew existed. In economy, the movie selection is generally a bland Hollywood selection, with Spanish on channel 1, and English on channel 2.