How do LISP macros differ from others?

I constantly hear LISPers boast of the macro capabilities of LISP. How are they different from, say, the macros one can make in C?

>>6,8
My Lisp code tends to be on average 2-4 times slower than the C version I would write and 10+ times shorter (most of the time). I think this is a very good trade-off for my needs. If I were to interpret it, it would be orders of magnitude slower. I tend not to use Lisp as a scripting language, but as a high-performance high-level language which lets me go low-level (it's a similar niche that SEPPLES attempts to fill, but unlike SEPPLES, my Lisp code is very generic and reusable, not to mention considerably easier to debug and maintain).