So depending on what websites/books you refer to–the number of families that make up this particular order can differ.
With the increase use of molecular genetics, organization of various animal groups (not just birds) have changed over the years.
So there are nine families (which include 71 different genera and approximately 450 different species) that make up the order Piciformes and/or Galbuliformes. The family Picidae (woodpeckers and their relatives) making up about half the total number of species.
There are two families that are either considered members of the Piciformes order, or members of their own order: Galbuliformes. Those two families are the jacamars (Family Galulidae) and the puffbirds, nunbirds, and nunlets (Family Bucconidae).
The other families that are included in this order (Piciformes) include the barbets. These birds are found in the tropics of both the New World and the Old World. Therefore there are actually now four different families instead of one: Capitonidae (the New World barbets), Semnornithidae (Toucan barbets, also found in the New World), Lybiidae (African barbets), and Megalaimidae).
The other two families are the toucans (Family Ramphastidae) and honeyguides (Family Indicatoridae).
This diverse grouping of birds can be found on all continents except for Australia and Antarctica; and only the woodpecker family is truly widespread outside the tropics.
The jacamars, toucans, and puffbirds are only found in the New World tropics; the honeyguides are found in Southeast Asia and Africa, while the barbets are found in both the New World and the Old World tropics.
One of the defining characteristics is that they have zygodactyl feet–this means that they have two toes facing forward and two toes facing backwards. This helps when they’re climbing tree trunks and moving around in the trees and shrubs.
These birds also nest in the hollow areas within tress. One interesting little tidbit–honeyguides are also specialized brood parasites (they will lay their eggs in the nests of other woodpeckers, barbets, and other birds).
One thing that has allowed for these birds to be so widespread is the differences in their beaks and feeding specializations.
The woodpeckers, wrynecks, and piculets all have a strong, sturdy, and chisel-like beak. This allows for them to chip away at the bark of trees. They then also have a long tongue that has barbs at the the tip, allowing for them to pull insects out of tiny crevices.
The jacamars have a long, slender, and sharply pointed beak. This allows for them to go after butterflies in flight.
The puffbirds have a stout and anteriorly compressed beak that is slightly decurved at the tip. This allows for them to go after large caterpillars.
The toucan has a very large and inflated beak, while the barbets have a stout, basically conical beak surrounded by bristles. Both of these birds go after fruits, though the toucan may also snatch eggs and young from other birds nests.
The honeyguide has a small and slightly hooked beak. They eat beeswax and bee larvae; after showing other animals (such as honey badgers) where the beehive is located.
Some photography goals for this order include: getting at least one picture of a member of every other family outside of the Picidae (woodpecker) family. A picture of a honeyguide leading a honey badger to a beehive would be really sweet to get (though that will again require a trip to Africa at some point).
References: https://bcs.whfreeman.com/webpub/biology/gill/birds-of-the-world/99010-29.htm; https://www.encyclopedia.com/scienec/encyclopedias-almanacs-transcripts-and-maps/woodpeckers-and-relatives-piciformes; and https://www.britannica.com/animal/piciform