M66 is also known as Arp 16. It made his atlas under the heading of spiral galaxies with detached segments. It also is part of Arp Arp 317. Making it the only galaxy to have two entries in Arp's atlas. M66 is a rather active galaxy due to interaction with the other members of its group. It is this interaction that likely accounts for Arp's "detached segments" seen in this galaxy. To me, they are just clouds of new, blue stars created by interaction with its neighbors.

NED classifies it as SAB(s)b;LINER Sy2, The NGC Project simplifies this to Sb while Seligman says SAB(s)b?. The galaxy's discovery is credited to Charles Messier on March 1, 1780. While some sources credit it's discovery to Pierre Méchain who did discover many objects in Messier's catalog that is unlikely in the case of M66. Messier was careful to note those found by Méchain but made no such note for this entry.

Arp's image: http://ned.ipac.caltech.edu/level5/Arp/Figures/big_arp16.jpeg
SEDS information: http://messier.seds.org/m/m066.html

14" LX 200R @ f/10 L=6x10, RG=2x10' B=4x10', STL-11000XM, Paramount ME