Hand-knotted rugs are woven one knot at a time on a loom, can take months to produce, and are the most durable and valuable construction — a well-made hand-knotted wool rug can last generations. Hand-tufted rugs are made faster with a tufting gun punching yarn through a backing fabric, then glued and finished with a canvas back; they're more affordable, but the glued backing can degrade and the pile can shed more over time than a knotted rug. Machine-made rugs (power-loomed) are the most affordable and consistent, with quality ranging widely by fiber and density — some power-loomed synthetics now closely mimic hand-knotted looks at a fraction of the cost and durability.
Wool is the standard for durability and stain resistance in natural fiber rugs and hides dirt well between cleanings. Natural fibers like jute and sisal are durable underfoot texturally but stain easily and aren't a good fit for high-spill areas like a dining room. Synthetic fibers — nylon, polypropylene, polyester — resist stains and fading well and are the practical choice for high-traffic or sun-exposed rooms, at some cost to the natural-fiber look and feel.
For a living room, the rug should be large enough that at least the front legs of every major seating piece sit on it — an 8x10 or 9x12 is standard for a typical seating group, not the 5x8 that looks proportional in a showroom vignette but small under real furniture. A rug pad (felt, rubber, or a felt-rubber combination) isn't optional for anything but a very small accent rug — it prevents slipping, adds cushioning, and protects both the rug and the floor underneath from wear.
A store that measures your room, or at least asks for real dimensions before recommending a size, is doing the part of the job that determines whether the rug actually works once it's home.