It has the widest variation of any hobby I know. From home-makers watching birds visit the scraps they put out in the garden, through to manic twitchers who will drive the length of the country to add another tick to their list.
Birders might watch wherever they happen to be, or stick to "their patch". They might have a favourite spot on the cliffs for seawatching, or they might like to "browse" reserves.
Personally, I've come late to the hobby - I've liked birds for most of my life, but only gotten into actually watching them for a couple of years. It can be an expensive hobby as well - quite aside from the petrol money, there are the binoculars, the scope, the clothes. If I bought all the kit from new, it would cost me hundreds, maybe thousands.
Fortunately, my dad has been into birds for years, and has enough disposable income to buy himself decent kit. He uses me to rationalise his buying - he can pass his old kit onto me, "to save me money". He's spent thousands saving me money. The only new kit I have are my bins (a nice pair of roof-prism 8x32s from the RSPB) and my books.
A good guide book is essential. Obviously, you choose the right book for your area, but I highly recommend RSPB's Birds of Britain and Europe by Rob Hume. My dad swears by the latest Collins guide.
You may also need more specialised books if you settle on a particular style of birding, or type of birds. I seem to end up at beaches a lot, so I also have Shorebirds of the Northern Hemisphere by Richard Chandler and Flight Identification of European Seabirds by Blomdahl, Breife and Holmstrom.
What you need most of all, though, is a patient sense of humour. You can spend all day in the field, see some lovely birds, have a great time, then you get home, tell your tales and somebody says "So, nothing new, then?"
Or the number of times you arrive at a spot already occupied by a group of birders who say "Oh, you just missed..."
The funniest thing, though, is the instant expert. The chap who decides he wants a hobby, something to get him out of the house. Half the fun for them is buying the kit, getting all the "right" stuff.
Last January, a few days after Christmas, I was at Minsmere Reserve. The place was stiff with middle-aged men with brand new equipment. Bins, scopes, even jackets.
I was in a hide, with a dad and his lad in at the same time. The boy, keen, pointed out of the window and asked what a bird was. The dad peered through his thousand-pound binoculars, frowned, set up a scope more expensive than my car, had a look, smiled, and declared; it's a duck.