Paris has two airports, Aéroport d'Orly, 16km (10mi) south of central Paris, and Aéroport Charles de Gaulle, 27km (17mi) to the north. Flights run daily to all parts of the country, but the high-speed TGV (train à grande vitesse) train services are usually more convenient. Charles de Gaulle is a major international hub, so you shouldn't have any trouble finding a flight, regardless of where you're flying.
There are six major train stations in Paris, each of which handles traffic to different parts of France and the rest of Europe. The most spectacular route is the Channel Tunnel (or Chunnel) between London (via Folkestone) and Paris (via Calais), a trip that takes only three hours. TGV services also link Paris with Amsterdam and Brussels.
Euroline buses run from Paris to cities all over Europe. Hoverspeed runs bus-boat-bus combos from London, but with the convenience of the Channel Tunnel routes you'd have to be pretty hard pressed to consider it. There are also ferries and hovercraft between Ireland and France.