I presume you mean the negative effects, so I'll try to explain those:

Addiction. Coffee contains caffeine, a rather strong chemical, and one that the human body doesn't naturally produce. Thus, one who drinks large measures of coffee (Exact amounts have not been published, but I believe one needs to drink multiple cups a day to reach this point) may find himself addicted, and may have problems whenever he can't/doesn't drink coffee, specifically severe headaches.
Sleep disorders. Caffeine can throw off the sleep cycle immensely, especially when consumed in large quantities. Caffeine wakes a person up by suppressing melatonin and blocking adenosine, 2 naturally produced chemicals which cause drowsiness and cause a person to fall asleep. With them blocked, a person will find himself more awake - but will also find it far more difficult to fall asleep.

Highly Recommended: Carefully monitor your caffeine intake: try not to consume more than 400 mg of caffeine a day (about 4 cups of coffee) and try not to drink coffee past 2:00 pm.

However, not all side effects of coffee are negative.

Here are just a few examples of positive benefits:

It helps a person stay awake and focused.
It can be healthy for you: "Studies indicate caffeine may reduce the risk for several different types of cancer, as well as lowering risks for heart disease, stroke and type 2 diabetes." (Michael J. Breus in his blog The sleep doctor)
It may help those who work out burn more calories, and may reduce muscle pain
It's been shown to help with mental performance; improving memory and reaction time, and increasing cognitive flexibility.

Hopefully this will help you consider if and how to consume coffee.

Note: This is a biased answer, I personally drink coffee and was just finishing a strong cuppa as I typed this. I did, however, try to base if on proven scientific studies.