There are many kinds of Deku. They all are sentient plants, supposedly related to the Great Deku Tree (but not all such plants are called Dekus, as is the case of the Deku Tree's children, the Koroks), distinguished by different forms and level of intelligence (such as the beast like Deku Babas). Firstly, lets focus on the Deku Scrubs. They are an ambiguous race and can't be classified as a whole. There are good, even respectable Deku Scrubs, but for the largest part of Hyrule's Deku Scrubs, they are greedy, feisty little guys that have at times sunk as low as to serve Ganon (in killing the Deku Tree, no less). In particular, the so called Mad Scrubs are crazed all out fighters that won't even hide in the safety of their nests when their enemies fight back.
However, under normal circumstances, the majority of the Scrubs will probably present themselves as Business Scrubs, trying to sell all manner of things to those who can manage to talk with them. That is, because typically business scrubs will shoot nuts to whomever walks by them and then hide in fear if the victim approaches. They say this is because they can't help but shoot nuts when they try to speak, but whether this is the truth or a convenient lie they use to get away with trying to rob strangers is hard to tell. The only thing to do with a Deku Scrub behaving like that, is knocking some sense into them. Those who did it, could witness that Business Scrubs can really get anything from anywhere; and they can be found selling things like potions in the deepest dungeons. Gibdo and ReDeads ignore Deku, perhaps because they lack meat.
In Hyrule only one settlement of Deku Scrubs has been sighted, deep in the Lost Woods (which, is really the natural habitat for them, although they are also fond of underground caves anywhere) in a grotto known as the Forest Stage. Here, they judge people based on their appearel, rewarding those they like and attacking those they do not.
In Termina, the Deku Scrubs have a whole kingdom deep in the swamp. In the palace, the massive Deku King presides over the daily affairs of his people with a short temper (a trait most Deku seem to share) and a firm, sometimes misguided, hand.
Dekus are varied in appearance. Deku and Mad Scrubs are mostly short, with massive round heads atop their little bodies, whereas Business Scrub tend to fatten up so that they appear to have no necks. Naturally they are made of wood, with leaves atop their heads (and flowers in the case of females). The more important among Deku Scrubs decorate their bodies with vines and clothes made of flowers, but most of them just go naked.
Deku Babas are man-eating flowers alike to wild beasts in intelligence, fortunately they are rooted where they're born. They have many subspecies one of which will even move in to their attacker if their stem is cut. Large carnivorous plants like manhandla may also be related to them, as certainly was the monstrous Diababa.
Deku and Mad Scrubs attack by spitting Deku nuts or (occasionally) by dropping them from the air like tiny bombs. In fact, similarly to the Koroks, Deku Scrubs can fly, but only for short distances and starting their flight from inside a Deku flower. Some Deku Scrubs can also perform a spinning attack or, if they know magic, attack by spitting bubbles.
Although their business can be very useful to travelers that know how to handle them, Deku Scrubs are not well liked. This should come as no surprise however because, as said, many of them are mean-tempered con artists at best and often downright wicked. Dogs hate Deku Scrubs and attack them on sight. Monkeys, who share the Deku Scrubs territory in Termina, also do not always get along with them.
The time when we apparently saw the worse of the Deku Scrubs was however when Ganon stole the Dark Mirror. A large number of Deku Scrubs became his followers, almost worshipping him. These Scrubs though were anomalous: they said Ganon awoke them from their slumber and that anybody who'd wander long enough through the Lost Woods Ganon created would become a Scrub. It appears therefore that at least some (if not all) of these were not natural born Deku Scrubs, but people that were turned into them. Whether this was or not the case, it is also unclear if some (or all) of these Scrubs were part of the Dark Tribe sealed in the Dark Mirror.
At any rate, though no good Scrubs were to be seen in Hyrule at this time, it appears unlikely that these unusual plain evil Scrubs can represent the condition of the average Deku Scrub, serving to that end much better the business Scrubs who still exist in Holodrum and Labrynna.