Invert sugar or belgian candi sugar is fructose and glucose, both monosaccharides. Table sugar is sucrose, a disaccharide, composed of one fructose and one glucose molecule. Yeast can really easily consume monosaccharides like fructose and glucose, but they have to split apart sucrose first to digest it. This added step can introduce off flavors and stress the yeast out. Using sucrose instead of invert sugar could give you off flavors in your beer and could take longer to ferment.
As awithrow mentioned, it is easy to make your own invert sugar by adding a little citric acid (I used lemon juice) to sucrose and heating it. The acid helps splits the sucrose molecules into their glucose and fructose components via hydrolysis. You can vary the color of the eventual sugar by how long you keep it at soft crack temperatures (275°F). I used this method while making a belgian quad and it came out awesome. However, the site I read recommended pouring it into aluminum foil to cool it, but when it cooled it stuck to the foil, so I would consider waxed paper like awithrow's site recommends.