Via Universe Today I saw this video time lapse of comet Lulin as it blows gas off its nucleus:
This was taken by Joseph Brimacombe, Southern Galactic Telescope Hosting, on February 22, 2009. The solid part of a comet is usually pretty tiny, just a few kilometers across. Bigger than a mountain, but a lot smaller than a moon. But a lot of that solid part is frozen stuff that turns into gas when the comet nears the Sun. It expands, forming a cloud around the solid nucleus. That cloud can be huge-- 100,000 kilometers across or more, bigger than planets!
The gas escapes away from the comet, forming a tail. That tail can get kinks, twists, sheets, ribbons, all kinds of shapes as it moves off. In the video, you really get a sense of the majesty and beauty of this process. As the tail fans out, sometimes it looks like we see part of it on both sides of the comet, but that's just a perspective effect. Imagine someone with long hair in the wind; their hair forms a comet-like shape behind them. Looking straight at them, face-on, you'd see hair on both sides of their head. That's more or less what we see with Lulin. You can see the so-called anti-tail in lots of the images posted of Lulin. Try a Google image search and be amazed. This is some gorgeous comet.