As previously mentioned, Barra is the airport where the planes land on the beach. It connects this most southerly of inhabited Outer Hebrides islands with a short flight to Glasgow, and a very short flight to Benbecula (for this, you can get a cheap day return ticket if you want to see a bit more of the Outer Hebrides).
Here's the beach, looking south towards the Airport terminal:
And here's a wider picture of the beach at low tide (both pictures by Flickr user Dowens):
It's worth getting on one of the flights in order to experience landing on the beach. We did the one from Glasgow, and it was spectacular; the plane flies quite low, especially over the hills. Then for the latter half of the trip, you fly over deserted islands and bits of rock with perfect beaches.
Here's the plane on the "runway" (picture from Ejbaurdo), awaiting boarding and departure from gate number 1 (of 1):
Alternately, you can watch the planes land and take off from outside the "terminal building", though you can end up with the complementary sand-blasting treatment. The building itself consists of a couple of rooms and an excellent little cafe that serves home-made chocolate cake. It's more fun waiting for the plane to land, as the truck zooms up and down, chasing the cattle off the beach/runway.