E12 refers to the base of the bulb, and is also known as a candelabra base. This is smaller than the "medium" base (E26) found on your average household bulb, like on a typical 60 watt incandescent bulb. Understanding the difference is easy when you know what the E and the numbers stand for.
"E" stands for Edison, who designed the "screw in" based that home owners are certainly familiar with. The number stands for the width of the base in millimeters. So a medium (E26) base is 26 mm across and a candelabra or E12 base in 12 mm across. You can see both styles in the decorative LED bulbs shown below.
Across almost all bulb styles these days, LEDs are becoming popular replacements for incandescent bulbs because of the similarity in look and features and the tremendous energy savings that come from LEDs. Many decorative incandescent bulbs run at 25 to 40 watts, with some using 60. LEDs operate on about 1/6 the power, so if you run a chandelier with 5 x 40 watt bulbs in the kitchen for 3 hours a day on average throughout the year, you're probably spending $25 to $30 on electricity each year just on that chandelier.
Meanwhile, replacing those bulbs with decorative LEDs like those pictured here would drive that cost down to around $5 per year. So as usual, LED bulbs will cost you more for the initial purchase, but within a couple years they should pay for themselves and, after that, put money in your pocket for years to come.