Liquids contain no bleach ... better for colours although a "colour" detergent is even better ... they can also accelerate the speed at which the machine will start to smell as there is no bleach to kill off stray bacteria (from the clothes) this includes the liqui tabs ...
Powders & tabs CAN contain bleach but do not always ...
Sensitive powders & tabs contain NO enzymes ... these are required to break up naturally occuring stains ... poo/vomit/grass/mud etc ...