Lighting is an important factor in a planted aquarium. It can be analogized as follows: light is like the accelerator of a car. The more light you have, the faster you are pushing down the accelerator. The less light you have, the less you are pushing down the accelerator. When you are driving faster, you must maintain your car more often, i.e. oil changes, checking your tires, and so forth.
Similarly, in higher lighting conditions, you will be "driving" your plants faster, and they will require more maintenance (i.e. carbon dioxide, nutrients, etc).
A guideline that can be used when describing lighting is the "Watt per gallon" (WPG) guideline. This is not a rule per se, but only a guideline (for reasons that will be explained below).
Lighting can be divided into 3 general categories:
1) Low lighting (~ 1 - 1.5 WPG)
2) Medium lighting (2 WPG)
3) High lighting (3+ WPG)
Correspondingly, plants can also be divided into these 3 categories, with "low light" plants being able to tolerate low lighting conditions, and "high light" plants requiring higher lighting conditions. This does not mean that "low light" plants cannot grow in higher lighting, however.
To explain why the WPG is only a guideline, one must consider the origins of it. WPG was originally designed for T12 fluorescent bulbs (i.e. bulbs that are 1.5 inches in diameter, the number after the T indicates the diameter in eighths of an inch). However, with the advent of various types of bulbs, i.e. T8, T5, power compact (PC) lighting, this guideline has become less accurate. In addition, this guideline cannot be applied in very small (10 gallons or less) or very large (maybe 55 gallons or more (ballpark figure)).
For instance, in my 2.5 gallon aquarium, I have two 13W compact fluorescents. This gives a total of 26 watts, corresponding to just over 10 WPG. This might seem suicidal at first (remember, the amount of light drives the amount of plant growth), however, because the WPG guideline is not accurate at small tanks, it turns out my 2.5g nano may fall under the "high light" conditions only (as opposed to "suicidally high light"). If I were to follow the WPG guideline strictly, and have 3 WPG over my 2.5g nano (i.e. 7.5 watts of light), then the tank would be poorly lit, and could only fall under a "low light" setup.
Not only is the amount of lighting critical to a planted tank, but the type of lighting is also important. As mentioned, there are several types of lighting available to the aquarist today (T12, T8, T5, PC, etc). Such bulbs can sometimes be confusing the novice planted aquarist. As a general rule, T8 and T5 are newer and more efficient than the older T12 bulbs. There are two camps of thought, one which prefers PC lighting while the other prefers T5 lighting, but to argue the points of these two types would be beyond the scope of this article.
One must remember that in general, fluorescent bulbs are all subject to a phenomenon called "restrike". In restrike, light that is emitted from the bulb will "restrike" the bulb, reducing the output of effective light. In general, straight fluorescent tubes are less prone to restrike than (say) the spiral power compact bulbs that can be purchased.
For some, the "colour temperature" of the bulb will also be a consideration. Plants in general are not too picky regarding the colour temperature, and some may argue that this point is not necessary, however, I have decided to briefly touch upon this.
In "normal" daylight, the sun is 5500 Kelvin (the unit of measure for colour temperature). Higher colour temperatures will appear bluer, while lower colour temperatures will appear redder. Here are some typical colour temperatures that can be found:
2700K "warm white"
3500K "cool white"
Do keep in mind that light is but one important factor in keeping a successful planted aquarium