Orchid roots have a number of unique adaptations, mostly to help with epiphytic growth habits, such as a whitish outer layer called velamen which absorbs water from the humidity in the air and helps the plant cling to tree bark, rock, or whatever else it may be growing on.
Actively growing orchid roots will have green tips. The white velamen layer follows a few days behind the root's growth tip. Healthy plants tend to produce slightly thicker roots that grow faster. You'll also see more numberous roots.
In some species, the roots branch frequently. This is most common in species with thin roots, such as many Oncidiums.
Epiphytic orchids frequently produce aerial roots that grow up and out of the pot. The idea is to capture moisture from the air, so it's a good idea to get such roots wet when watering orchids, and when misting the plants for extra humidity. You'll see the velamen turn green when wet.
Some orchids, such as many terrestrial orchids and Paphiopedilum orchids have hairy roots when healthy. I assume this is to help the roots collect more moisture by increasing surface area and the amount of contact with the growing medium.
As a general rule, orchids don't like having their roots disturbed or damaged when being repotted (or, if grown in the garden, when being transplanted). Some species are much more finicky about this than others, though.
Most orchids like a lot of air to their roots, so letting the roots sit in water is a bad idea (they'll die). There are exceptions to this, such as Phragmipedium and Disa, but it applies to the vast majority of orchids. Many orchid potting mixes and orchid pots are designed to permit lots of air to get to the roots.
If a plant isn't doing well, try unpotting it and taking a look at its root system. If the roots aren't healthy, now you know what's going on. Remove any dead roots and consider repotting the plant in a new orchid potting mix or changing your watering frequency.
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