The idea that laundry needs to be separated is integral to most rules of fabric care. There are a number of reasons to separate laundry. Primarily, separating laundry is about ensuring that each type of fabric receives the care that it needs. Separating also helps to reduce color bleeding, and will ensure that the laundry is evenly distributed with similar laundry in the dryer, so that it will dry more quickly. If you absolutely cannot separate laundry, launder it with cold water and a cool dryer setting to reduce the potential for damage.
At the very least, it is an excellent idea to separate laundry by color. Doing a separate white load allows you to bleach whites, keeping them crisp and evenly colored. Whites can also be washed in hot water, which will help to remove odor causing bacteria and mold. Separating by color also prevents bleeding. Try to keep darks together, and wash them on cold. Fabric which has not been washed before should be washed separately, in case it is not colorfast.
Bleeding happens when a dye is not fixed to the fabric. Being sluiced around in the laundry water pulls the dye out, resulting in pink socks, blotchy clothing, and similar familiar but unfortunate laundry accidents. Washing on cold can reduce bleeding, but keeping similar colors together also ensures that a small amount of bleeding will not matter. When you separate laundry, try to divide whites, blacks and blues, pastels, and vivid colors like red.
Another reason to separate laundry is weight. An imbalance in the washing machine can damage it, and the unequally weighted fabric will also take longer to dry. Heavier fabrics like terrycloth, used to make towels, should be washed separately from lighter ones such as cotton and linen. The heavy fabrics can also entangle with the lighter ones, potentially causing damage as they pull at delicate fibers.
It is also wise to separate laundry by type. Washing linen with other linen, for example, means that you can be confident about the washing and drying cycles, because you know that the entire load of laundry requires the same fabric care. It is not always possible to separate laundry by type, and if it means doing a load with only a few garments, it is better to wash similar fabrics together, rather than doing two separate loads. Also, colorfastness is the most important issue to consider when you separate laundry.
Learning the cycles on your washing machine will also help you do laundry more effectively. Most machines have a cycle for delicate fabrics in addition to a regular cycle. You can choose water temperatures and the length of the wash. Some washing machines also offer presoaking and other features which can make doing the laundry easier. Learn about the features on your washing machine to use them to advantage, and do the same with dryer.