Gel pens are a relatively recent product, but have very quickly become a favourite for lots of people. They are very similar to liquid ink rollerballs, but the ink is in the form of a gel. Like ketchup or 'solid emulsion' paints, gel ink is solid stuff until a force is applied to it - then it becomes liquid. The act of touching the ball against paper is enough to change it, and make the ink flow. As soon as the ink is on the paper, with no force applied, it's solid again.
The gel can hold pigment, so the ink is often more permanent than liquid inks, and can write in pastel colours that show up on dark paper, or even have sparkling fragments of metal or cosmetic-grade glass for sparkling or metallic effects. Combined with the amount of ink these pens put down, this means gel pens usually make very bold, bright lines.
The disadvantages of gel pens are that they can tend to skip with very light pressure; the ink doesn't last as long as the same sized ballpoint refill would; and they don't write on all surfaces a ballpoint will, so shiny paper (like greetings cards) may be a problem. Fans of gel pens consider these small prices to pay for the smoothness, and the bright colours and really black blacks.
Pilot have a huge range of gel pens, including the classic Pilot G2 retractable, with lots of choices of tip sizes and colours. Their G-Tec-C4 can write an amazingly fine line with surprising smoothness, with ten colours available. Many of Uni-ball's gel ink pens use their 'super-ink', which is highly permanent.
Monteverde's gel refills are also worth mentioning - they'll let you use gel ink in quite a few pens that would otherwise only have ballpoint refills available.