Minecraft wiki entry for Caves_%26_Cliffs, associated text: A light source with a light level of 3.
Up to four can be placed in a single block, similar to sea pickles. If 1 more candle is placed in a block, it increases the light level by 3, making a max light level of 12.
Can be lit with a flint and steel, fire charge, an arrow fired from a flame-enchanted bow, or a fire charge shot by a dispenser if it hits the candle.
Have variants for uncolored and all 16 colors.
Uncolored candle is crafted with 1 string and 1 honeycomb.
Colored candle is crafted using one of the 16 dyes and one uncolored candle.
Right-clicking a candle on an uneaten cake makes a "birthday cake", which can have a single candle placed on it.
Example:
If the cake is eaten, the candle pops off.