Hi Keith. Plastic sheet, often called plastic card in the UK, is simply that - sheets of card-like plastic, very often white in colour, and available in various thicknesses, or gauges, up to about 2mm, which is thick enough to build houses with!
Most modellers will use the thinner varieties, some roughly the thickness of copier paper, another more or less the same as the card found on good quality greetings cards, and some more like a stiff mounting card.
It's used for virtually any application in modelling you can think of, from scratch-building, for example, floors and bulkheads in aircraft models, making boxes or 'boxing-in' wheel wells and gun bays to filling cut-out areas, It can be easily cut or scored, and is therefore useful for cutting into small strips to fill or bridge gaps, although strips are normally used for this work. Plastic strip is just that, the same gauges of plastic, cut to mvarious width strips, normally about 10 to 12 inches long.
Manufacturers such as Slaters, and Javis in the UK supply these materials to the model and hobby shops, as well as 'Evergreen' and 'Plastruct', who also make other plastic structural materials, such as 'H' beams, angle strip, tubes and rods etc, mostly used in the archtitectural model making industry, by design studios and film company model makers, as well as a great many scale modellers.
Most of the UK-based mail order model comapnies will list, and supply, plastic sheet and the other products mentioned above, as well as embossed sheets, representing paving, roof tiles, brick and stone walling, cobbles etc etc., the latter products normally used in model railways.
Hope this answers your question mate.