All green plants require light to perform photosythesis, during which special cells convert the energy contained in the light into sugars. In the process, plants give off oxygen.
Plants vary widely in how much light they need to thrive — from low-light houseplants, such as dracaenas, to fruiting edibles, such as tomatoes, that need much brighter light.
Seedlings require lots of bright light, and when they don't receive enough they get weak and leggy. In most cases, even the sunniest windowsill won't provide the intensity or duration of light they need, especially in winter.
When you shop for grow lamps, you will notice that they are marked with numbers like 2700K or 4000K. This refers to their relative warmth or coolness in the color spectrum - the higher the number, the cooler the light. Foliage growth is generally best around 6500K, although many plants need a period of warmer light, around 3000K, to produce flowers and thus fruit.
In other words, if your goal is to simply produce plants, green leafy vegetables or root crops, you need a higher color temperature. If you want to grow flowers or any fruit plant (cucumbers, tomatoes, peppers, lemons, etc.), you also need lamps with a lower color temperature / spectrum. Some types of luminaires / lamps can be found as in full spectrum, to simplify the choice.