It is a fluid composed of living and dead bacteria, dead fixed and free cells (latter representing body's phagocytic response) and foreign material such as sutures, implants and splinters.
Color of the pus may give a clue regarding the organism.
Creamy - staphylococcus
Watery opalescent - streptococcus
Blue /green - Pseudomonas
Purplish brown - Amoebic liver abscess
Yellow granules - Actinomycosis