Holodeck matter, also known as holomatter, is a partially stable substance giving the illusion of solid matter, held together by force fields created by hologenerators.
Outside of the range of holographic projectors this substance loses cohesion and quickly dissipates into energy. Within range, this substance can have all the properties of "real" matter but is controllable by complex computer software.
Holograms are typically combinations of photons, force fields, and matter (both "real" and holographic) used in degrees of complexity depending on computer safety settings. (TNG: "Encounter at Farpoint", "The Big Goodbye", "Elementary, Dear Data", "Ship in a Bottle"; VOY: "Phage", "The Killing Game", "The Killing Game, Part II", "Extreme Risk"; et al)