What is meant by prokaryotes ?

All cells and complex cell having creatures fall into three domains. But two of these domains are similar enough physiologically that we lump them together, so we say that all living creatures can fit into two main super groups (or empires if you're oldschool). All multicellular complex organisms are eukaryotes. But prokaryote doesn't just mean unicellular:

Prokaryotes don't have a nucleus, and are smaller in size. They also lack actin and tubulin protein structures. The cells are covered in peptidoglycan and for archea, a protien like substance. All lack cellulose, chitin and pectin. They reproduce by binary fission and have circular DNA, no separate chromosomes, and no diploidy. They also lack membranous organelles (no mitochondria no chloroplasts). They also cannot truly phagocytise and endocytise material like eukaryotes.

The prokaryote group is broken into archaea and bacteria, the two domains that have all the above in common. They have different membranes, live in different conditions, and archaea are, weirdly enough, closer in RNA to eukaryotes. But don't worry about that.