They're similar in shape and size and colour to twigs and they're flavoured with Marmite. Officially I think they're made of wheat or some such nonsense like that, but actually they look exactly like spring twigs. They taste better though.
Twiglets are one of the best foodstuffs ever made. And now I've come back to this thread and am really pissed off that I went to the shop to buy pesto and didn't buy any Twiglets.
Twiglets are also the source of one of the best jokes in Spaced.
TIM- Ooh, I feel like shit. What did I do last night?
DAISY- You ate your own weight in Twiglets and punched an artist.