To hoard is to squirrel stuff away, like gold bricks or candy wrappers. A horde is a crowd of people, usually, but it can also be a gang of mosquitoes, robots, or rabid zombie kittens.

If you gather all the info you can about hoard, and store it away for later, you'll find it comes from the word for "hidden treasure." When you hoard something, you are collecting lots of material, usually of value, in secret. You store these things in case you need them later. It's a noun and a verb. Hoarding canned goods and batteries before a hurricane is smart. Not throwing out that hoard of old playground equipment in your yard, not so smart. Here's some hoarding from the news:

American firms continue to hoard cash and overall bank deposits soar despite rock-bottom interest rates. (New York Times)

Clippings and advertisements for free samples were hoarded and quickly posted. (Lauren Ann Isaacson)

Every one is given at least one talent for use; not to hide and hoard away. (Louise Vescelius-Sheldon)

A horde, on the other hand, is a busy mob, like the one that chases Frankenstein's monster with torches. Hordes are often roving and mad. Horde is usually derogatory and should be use with care. Here are some hordes from the news:

In China, it means angry hordes parading victims wearing dunce caps through the streets before stringing them up in public squares. (Time)

As darkness fell, women illuminated by wood fires stirred vats of couscous and beef stew for the hordes of visitors. (New York Times)

The only reason people get these words confused is that they sound. Remember, there's an "a" in hoard and "gather." Horde is just a wild bunch of letters holding pitchforks.