This is the most basic questions which people will ask. Lets straightly answer to the question.
You guys might have came across multi-colored hologram sticker which changes colors when we change the position. These Holographic stickers might be sticked on a mobile battery, on a debit card, or other kinds of products.
By definition, a Hologram is nothing but a 2d / 3d image which changes its color when we view the image from various angles. The image gives us a rainbow colored visual effect when it is exposed to light.