They are different. A quick way to check is to look if there is a letter or a number – Metro uses numbers and RER uses letters.
Metro: Métropolitan chemin de fer (Metropolitan railway): A train system running mostly underground, only in central Paris. Trains cover short distances between stations and have a frequent, but unscheduled, service – similar to the London Underground. One single fare zone.
RER: Réseau Express Régionale (Regional Express Network): A scheduled commuter train system, most of which is above ground except within Paris. There are 5 lines (A through E) and it covers most of the Île-de-France area. Trains cover longer distances than the Metro and have fewer stops; routes are usually faster. Separated into fare zones (1 – 5). If you get a train from Charles de Gaulle into Paris, you take the RER B train; if you go to Disneyland Paris, you will take the RER A.
When you purchase a Paris Visite Card, it allows for travel on both within the applicable fare zone.