Paris is not a station. It is six of them. London is not an airport — it is five of them. Booking "Paris to Rome" gets you a different journey depending on which station you arrive at, and which airport on the Rome side. Travellers who have not lived in Europe routinely book the cheapest fare without realising it lands them at Beauvais (1h15 from central Paris) when they could have arrived at Charles de Gaulle (40min) or Orly (45min) for €20 more.
The Paris stations. Trains arrive at: Gare du Nord (Eurostar/Thalys/local north), Gare de l'Est (Germany/east), Gare de Lyon (south/Italy/Switzerland), Gare Montparnasse (south-west/Brittany), Gare d'Austerlitz (south-west secondary), Gare Saint-Lazare (Normandy). Each is in a different arrondissement. Metro transfers between them are 15-40 minutes.
The Paris airports. Charles de Gaulle is the main hub (RER B to centre in 35min, or €60 taxi). Orly is the budget hub (Orlyval + metro 45min, taxi €50). Beauvais is Ryanair's "Paris" — actually 80 km north, 1h15-2h bus to centre.
The London picture is similar. Heathrow is 15-25min by Heathrow Express. Gatwick is 30min by Gatwick Express. Stansted is 45-60min by Stansted Express. Luton is 30-45min by train. London City is closest at 10min DLR. Eurostar arrives at St Pancras International — already in the city.
Practical takeaway: when comparing flights or train fares, check what airport/station code is in the booking. CDG ≠ ORY ≠ BVA. STN ≠ LHR ≠ LCY. A €40 saving on Beauvais vs Orly evaporates the moment you realise you're paying €25 for the shuttle bus. A €20 difference between Roma Termini and Fiumicino Airport changes whether you arrive at the city centre or take a 30-minute train into town.
The boring rule that works: prefer the station/airport whose name matches the city. Paris-CDG over Paris-BVA. Roma Termini over Fiumicino if you're optimising for time, but Fiumicino if you're connecting onward to a flight.