Refers to anything pertaining to hardware. The opposite of physical is logical or virtual, which describe software objects. For example, physical memory refers to the actual RAM chips installed in a computer. Virtual memory, on the other hand, is an imaginary storage area used by programs.
A physical data structure refers to the actual organization of data on a storage device. The logical data structure refers to how the information appears to a program or user. For example, a data file is a collection of information stored together. This is its logical structure. Physically, however, a file could be stored on a disk in several scattered pieces.