The RER speed line is a subway/train hybrid - in town trains go underground, out of town they arrive to the surface to turn into ordinary suburban trains. Within Paris (zones 1-2) you can go by RER with an ordinary ticket.
RER lines go to many Paris suburbs, including EuroDisneyLand. On the subway scheme RER lines are in bold. Their full routes aren't shown due to missing space - only directions (but you can get the RER scheme at any subway office). RER's 4 lines have code letters - A, B, C or D. The letters are often accompanied by numbers as the lines usually divide - so mind it while getting on a train.
RER tickets are 1-way only. Never throw them away since they let you out of its stations. RER trains have 1st-class cars in the middle. RER tickets are valid for all the city subway. 1 subway ticket is valid for any station no matter its zone (for example, from Pont Neuf (zone 1) to La Defense (zone 3) on line 1). But using RER, you have to pay extra for zone 3, otherwise you can't pass turnstiles out.