The short answer is: the Word of Wisdom is a health principle with a promise provided by the Lord to the Prophet Joseph Smith in Doctrine & Covenants 89 for the physical and spiritual benefit of His children.

A principle is an enduring truth – a law, a rule you can adopt to guide you in making decisions. Generally, principles are not spelled out in detail. That leaves you free to find your way with an enduring truth, a principle, as your anchor.

–President Boyd K Packer

The slightly longer answer from the LDS Church Handbook:

The only official interpretation of “hot drinks” (D&C 89:9) in the Word of Wisdom is the statement made by early Church leaders that the term “hot drinks” means tea and coffee. Members should not use any substance that contains illegal drugs. Nor should members use harmful or habit-forming substances except under the care of a competent physician.

–Church of Jesus Christ of Latter-Day Saints, Church Handbook (2010)

But when I get asked this question, the subtext is generally along the lines of: What is ok for Mormons to eat and drink and what is not? Why is chocolate ok to eat (which contains caffeine) but coffee is not? Why can’t you drink wine or beer? What about non-alcoholic wine and beer? And what’s up with black tea (with caffeine) versus herbal caffeine-free tea?

Here’s some clarification based on the research I have done for my family.

Definitely okay:

• Hot Apple Cider
• Hot Cocoa
• Caffeine-free Soft Drinks. My favorite is LaCroix Pamplemousse (Grapefruit) Flavored Sparkling Water w no sweeteners or calories.
• Chocolate aka Mormon Medicine (because it cures all that ails you!).
• Moderate Quantities of Meat
• Postum (which is fine from the perspective of Mormon orthodoxy, though it should be avoided with a bargepole from the standpoint of good taste).
• A Diet Rich in Grains and Vegetables.

Probably okay:

• Herbal Tea (according to the Word of Wisdom, herbs are to be used with prudence and thanksgiving). The only kind of herbal tea I will drink is medicinal, generally to soothe sore throats (Licorice-Echinacea), upset tummies (Peppermint), and calm the mind before bedtime.
• Cooking with wine, because the alcoholic content burns off during cooking (note: the same doesn’t apply for coffee. See below). Some very conservative Mormons, however, won’t use so much as a teaspoon of vanilla extract in a batch of chocolate-chip cookies!

Possibly okay:

• Nonalcoholic Beer and Sparkling Cider. Personally I do not drink either, nor would I ever serve them, unless I was helping someone reduce their alcohol intake (eg a convert or a member who might be struggling with the WoW principles).

Probably not okay, but no one knows for sure:

• Decaffeinated Coffee. A June 1988 article in the official Church magazine never said that decaf is forbidden. On the other hand, one of the questions asked by both Stake Presidents and Bishops during the Temple Recommend interview is adherence to the Word of Wisdom and they aren’t supposed to deny a member a Temple Recommend for drinking decaf. Apostle John Widtsoe advised members that consumption of decaffeinated drinks is not against the Word of Wisdom.

Definitely not okay:

• Alcohol, including wine and beer
• Black tea, green tea, and other caffeinated teas
• Coffee and recipes that use it (include desserts like Tiramisu and Red-eye Gravy, a popular breakfast sauce served in the South)
• Iced coffee and iced caffeinated tea
• Illegal drugs, recreational drugs, and illicit prescription medications
• Tobacco.

The subject of endless debate: Caffeinated soft drinks.