This might seem obvious but since many (most ?) indoor plants like it best, it is important.
East, South, and West facing windows give the brightest light. South and West though may be too hot for some plants.
Many plants will get burnt or too cold if they are placed directly on the window sill. A sheer curtain is a perfect way to mitigate the harshness of direct sunlight.
Likely most plants will give warning signs if the sunlight is too hot or direct without dying. White, Straw-colored, or brown spots might appear on sunburnt areas.
Variegation and other desirable foliage colors will be more vibrant when the plant does get enough light.
Plants that don't get enough light will get 'leggy', might turn a deeper green or lose it's variegation, and grow at a slower rate and might eventually die.
Epiphyte
A plant that grows on other plants and depends on them for support but not water or food. Tends to be more neutral than a parasite or symbiotic plant.
Some are also called "air plants" because they don't tend to grow in the ground and some get their water not from their roots but from their leaves.
Examples include: Orchids, Spanish Moss, and Bromeliads.
Phototropism
The term for plants' ability to grow towards sunlight and is a nifty adaptation.
Phototropism is caused by a growth hormone that decays in sunlight so the side that is more shaded will grow more quickly and cause the plant to lean more towards light.
Without phototropism, a plant will just grow upwards, away from the pull of gravity.
Stoma
Tiny holes in the surface of a plant that allow it to 'breath' and 'drink' water. Stoma can be opened to allow for intake of carbon dioxide and release of oxygen. Some plants will keep their stoma closed during the day to retain moisture.
Annual
A plant that lives only for one year. Example: tomatoes, petunias
Biennial
A plant that lives for two years. Some biennials won't produce flowers or seeds until their second year. Example: Pansies, foxglove, sweet william, columbine
Perennial
A plant that lives for many years. Example: sedum, russian sage, peony, lavender
"Nip it in the bud"
This idiom originates in plant cultivation. To inhibit undesired growth, the bud of a plant can be nipped off. This type of pruning can promote bushier growth or a healthier main growth, and can be used to kill a seedling.
More Information
Common Pests and Diseases
Spider Mites
Scale
Mealy Bugs
Fungus Gnat
Root or Stem Rot
Root rot is a very common way that house plants die. It has many causes:
Over watering - This is very easy to be guilty of but also very easy to fix. Many plants prefer the first inch or two of their soil to dry out before being given more water
Soil that is heavy/doesn't drain well - Soil will drain better with more air pockets and some additives. When potting a plant, don't squish down soil but allow for water to level it out instead. Pearlite, bark, and sand can all help with draining and what's needed will depend on the type of plant
Container with insufficient drainage - Containers should have at least one effective drainage hole. If a container is kept in another more decorative container or saucer, make sure to dump the water that pools at the bottom to keep soil and roots from getting swampy.
Humidity is too high - Terrariums can be a very attractive way to keep plants but depending on setup and know how, it can also restrict air circulation. Too much humidity can lead to rotting.
Fungal Leaf Spots
White Mold
White mold will look a bit like moldy food and the cause is the same: humidity. If the mold is just growing on the soil, it in and of itself won't kill the plant but can be a warning sign for problems that will.