What is trisomy 18?

I'm guessing that most people who read this will know this already, but we might as well start with an overview.

Trisomy: "an abnormality characterised by the presence of an additional chromosome to the normal diploid number". Chromosomes are the components of cells that carry the person's genes - often likened to striped socks, where a sock represents the chromosome and the stripes are the genes.