Least maintenance and most durable is level loop (often called “Berber”). The loop construction provides 4x the surface of wear, hence it's durability. It doesn't show tracking at all, and now comes in patterns and colors for every taste. Keep your carpet neutral and consistent throughout for greatest flexibility and resale. Use color in rugs, they are personal and move with you, carpet doesn't
High twist cut pile, also known as frieze, is considered trackless. That is it doesn't show vacuum and foot prints as easily as a regular cut pile. Qualities vary.
Velvet plush cut pile while certainly the softest underfoot, shows every mark, foot print, pet paw print, and vacuum tracks. It is very formal in nature and maintenance intensive.
Shag is back! Fun in a contemporary environment, but watch the depth, it isn't rakeable anymore and can still swallow pet turtles.
Rugs are appropriate for every room and over wall to wall, not just hardwood. They will anchor a furniture grouping and add interest and softness. They are truly artwork for the floor.
For size, the rule of thumb is that it should have all your furniture on it (such as a 9×12 in a family room) or all your furniture off it, (such as a 6×9 in the same room) the obvious exception being a coffee table or ottoman. Try to avoid the furniture resting half on/half off. Most dining rooms require at least an 8×10 rug to avoid the chairs “falling off.”
Smaller rugs make great accents and you aren't paying to cover them up with furniture! Be careful though, substituting a 4×6 for a 6×9 will leave you looking like you have a postage stamp on the floor.
Rugs can also make a significant impact on acoustics in rooms with lots of windows and vaulted ceilings. They can even be changed seasonally for mood shift!
Runners are an important consideration not just for aesthetics but for safety and acoustics value. On stairs noise often is amplified, a runner will muffle the traffic sounds. In hallways, runners serve to breakup a tunnel like effect with pattern and warmth.
Be sure to buy the proper pad for your rugs. Pads extend the life of the rug, as well as protecting the floor beneath it.
For hardwood and tile floors you'll want a superloc pad which has a rubberized backing on one side, and a thick jute/felt pad on the other. Do not use the waffle type pads, they can leave marks requiring refinishing on hardwood, and the superloc will even out any tile variance. For vinyl floors, a waffle pad or other slim line pad is acceptable.
On carpet a miracle hold pad or a sticky back pad is recommended. This pad has adhesive on both sides, one to adhere to the carpet, the other to stick to the rug, preventing the age old challenge of creep in the rug, and giving an extra layer of protective backing to prevent any bleed through of colors from rug to carpet.
Most reputable rug dealers will offer a stain guarding program. Some offer it free of charge with every purchase. Forcefield is the name brand of choice. It doesn't actually prevent stains, no stain guard does, it does give you considerable extra time to clean up, as it provides a protective shield on the surface forcing any spillage to bead up rather than be absorbed.