With physical partitions, a disk is subdivided into partitions. And somehow the disk driver knows about the subdivision scheme. The driver may read a table off the first sector of the disk. Or the driver may just get the model number of the drive and pick a built-in table. The special files are named to indicate the section. c0t0d0s2 would be section "2" of the disk (which was always the whole thing).

With logical volumes, a disk like c0t0d0s2 (or maybe just c0t0d0 theses days) is just a physical volume which gets tossed into a pool called a volume group. Then you have very great flexibility in creating logical volumes. A logical volume may be a very small slice of a disk. Or it may be so large that it is several disk drives.