Palm was originally founded by Jeff Hawkins and Donna Dubinsky and, after selling out to 3Com (a company that could never quite figure out the handheld business) they moved on to found Handspring (a Palm OS licensee).
After the disappearance of the Newton, Palm PDAs built their own market segment, mostly unchallenged until the emergence of the Pocket PC (which, as usual in pre-2000 Microsoft designs, tried to shoehorn a full GUI into a different form factor).
There were quite a few Palm partners/licensees, of which the most notable was probably Sony (with the Clié series - often thought to be the very best ones, of which the PEG-NX70V is a prime example).
For further info on licensees and devices, I recommend taking a look at the Palm Evolutionary Tree, which lists all known Palm-based devices up to 2005.
These days, thanks to the shift towards smartphones, Palm is mostly about the Treo range, and have even adopted Windows Mobile as an OS.