Please note: Many plants that were previously classified as Calathea have been moved into the Goeppertia genus. I have not yet updated this page to reflect those classification changes!
Calatheas are members of of Marantaceae family, which contains all prayer plants. Calatheas are grown for their foliage, which ranges from dark green and velvety (Calathea warscewiczii) to light green with detailing that appears hand-painted (Calathea musaica). Calatheas are more demanding than many of the other plants profiled on this website, and are not the best choice for beginners. However, given that the most common cause of houseplant death is overwatering, and that calatheas like to stay on the moist side, I personally believe that the difficulty of growing calatheas is overstated for the typical grower who over-showers their plant with affection (and water).
Note that stromanthes and ctenanthes, two other genera in the Marantaceae family, are often mislabeled as calatheas, and calatheas are often mislabeled as marantas. I don't believe I've found any retailer that reliably labels their prayer plants. These plants will often come unlabeled from wholesalers, and nursery personnel will often just google to attempt to identify the plants they have in stock. Mistakes are common, so always do your own research to identify your new plant.
I do not suggest allowing calatheas to get any direct light. While they might appear to thrive at first - especially if you improve other variables, like water, from their nursery habitats - they will eventually sunburn, dry, and struggle to keep new growth healthy. Direct light is especially deadly if your plant is not staying adequately moist. Aim for moderate indirect light.
Calatheas need to stay moist, but not wet, at all times. (Don't know the difference? Get a moisture meter! If my calatheas are toward the bottom of the "moist" range in the top couple inches of soil, I water them.) If your potting medium drains very poorly, you might struggle to decide when to water, as the middle of the tight root ball may almost never dry out. My advice: Repot into better medium (see below).
Many growers say that calatheas (and all prayer plants) seem to do best with distilled or filtered water, and that minerals from tap water will tend to result in the leaves getting brown and crispy. I haven't found great evidence for this claim with a cursory search of scientific literature.
I do not suggest keeping calatheas in terra cotta pots, especially for inexperienced growers, unless you really cannot restrain yourself from watering the plant multiple times a week. The upkeep of keeping a calathea moist in terra cotta seems overly burdensome for the average grower.
To keep your calathea looking its best, you really need a humidifier. These plants do best for me above 55% humidity, which is just not achievable in most locales (especially when you take into account the drying effects of central heat and air conditioning) without a humidifier. There are good humidifiers on the market for $30 or less that will help keep your (less be honest, somewhat pricey) plants in tip-top shape.
Even though calatheas like to stay moist, I still keep mine in well-draining soil and repot out of the nursery soil as soon as possible. Calatheas can still experience root rot, and since their roots get quite dense, I find that the soil nurseries sell them in can stay soaking wet almost indefinitely.