Hmm, one of my best friends is a Druid and I guess I can compare and contrast. There are traditional druids and witches that have nothing to do with any neo-pagan group or organization. In this case, my friend is a traditional Druid... That's to say his Gods are the Celtic ones and his ways are not what you find in the books on Amazon. Most of what Quintessence has said is correct for modern neo-Druids... So, no need talking about that.
There are many many MANY more traditional Druids in particular than affiliated with organizations.
First, Druids generally don't believe in The Threefold Law/Wiccan Rede since they will often feel hostile to forces and people whom they view hostile to nature and have no problem winging a few curses at them.
Second, they are closer to traditional witches than Wiccans because of the first case. The difference is Druids venerate nature gods, but traditional witches probably see all spirits and gods the same. Animism is a prominent feature of both.
Third, Otherworld -- basically the realm of the dead, spirits, elementals, and all the fun little critters. Generally, Druids and Trad Witches believe in this... Wiccans probably do not, or do not know and aren't steeped in the folklore very much.
Fourth, the Gardner junk about elemental correspondences and all other supposed required tools and whatnot are not used either by the Druids or Trad witches. Both have the view that the elements permeate everything -- directional correspondences are rubbish. This changes every ritual and working for both.
There are other points, but sort of drilling it down..
That being said... Modern Druid organizations are just a bunch of people that do the "Awen" thing, and Traditional Druids have nothing to do with Wicca.