In the books the Tripods have several tentacles, presumably three; in the series, only one. Even that was problematic on-screen: in the late 70s, Douglas Adams's edition of the Doctor Who guide for writers specifically ruled out any kind of tentacle as being impossible within the budget.
Otherwise, the Tripods look very like the book's first mention of them:
...the great hemisphere of gleaming metal rocking through the air above the three articulated legs, several times as high as the church.
The books also make reference to portholes of some kind, which may or may not be eyes, to tentacles and to a central opening cavity under the belly: all of which appear in the series, though the Tripods have only one tentacle each, and aren't quite so casual about swishing them about. As for movement:
It was Richard Bates who pointed out that it is very difficult to move on three legs without simply going round and round in a spiral.
(John Christopher). They tried digging out the 1953 movie of The War of the Worlds, to see how the Martian machines had been realised on the big screen, but it was no help -- Hollywood had transformed them into flying saucers. But the BBC could hardly follow suit, or make The Quadrupeds instead; and the end result is a jerky, menacing movement, favouring one leg over the others. These articulated legs show some similarity to the same modelling team's earlier work, such as the Jaggaroth spaceship in the Doctor Who serial City of Death.
[Jaggaroth spaceship, 1979; Tripod, 1984]
To make us think the model Tripods are 20 meters tall, the series shows us either huge legs (which crush houses, have horses running between them, and so on) or distance shots. Sometimes they are more cunningly introduced into the main image, for example by shadowplay on the schoolroom window in 1.1, using the then newly invented Quantel Paintbox. The program-makers used three sizes of model: 30-foot high full-scale models of the bottom half of the Tripod legs, 10-foot high Tripod tops (with the bottoms of the legs missing) and 2-foot complete models, for the walking animations.
All Tripods are identical in the books, and therefore impossible for the humans to tell apart, but in the series they come in two varieties: the more common silver ones and a reddish, more angular version like a warrior ant, armed with lasers. This latter type first appears, quite dramatically, in episode 1.12. The scripts called for a third, black variety to appear in a similar coup de théâtre in series 3.
[Warrior Tripod and shaved, newly-Capped head]
The Cap is, according to the book, a silver mesh somewhat like a medieval helmet, surgically joined to the skull. On-screen, these appear as much finer workings in gold, plausibly like conductive circuitry, and moreover are triangular. The image above shows a Cap newly fitted to a head shaved bald for the occasion: for the most part, one only sees a glint of gold through a Capped person's hair.
The designs extended a little further than simply the models. Typical of the care for detail in the series is that at one point a group of Vagrants, obsessed with their own failed Cappings, are shown to have built wooden imitations of both a Tripod and the Caps, which they use in strange rituals.