Something that is becoming more common is small breed dogs with coats so matted they need to be shaved back to the skin or given a poodle cut.
If you have a new pup and it has long fur get it used to being brushed from when it arrives.
If this is not an option find a dog groomer, and have it done regularly.
It is just like a hair-cut for us humans.
Toenails too, grow fast and whether the dog is small or large and if they are not being walked regularly the enails need trimming.