Grout is the substance used to fill the spaces between floor and wall tiles. A mixture of water, cement, sand, and color tint, it is a fairly porous substance that, over time, can fill with dirt, mold, and other unpleasant contaminants. Proper grout sealing can help to prevent these problems from occurring, but even the best sealing job may not be enough to prevent grout stains from appearing over time. Fortunately, cleaning your grout, while sometimes tedious, is fairly easy work.
Grout cleaning should begin with the cleaning solution itself. In most cases, grout can effectively be cleaned with do-it-yourself cleaning solutions, such as a mixture of equal parts baking soda, salt, and water. Spread this cleaning solution over any dirty areas and allow to soak for a few minutes. Then, with a stiff brush, scrub the area clean. You may also want to consider applying a bleach solution if there is any mildew or mold in your grout, as bleach will kill the spores and prevent them from growing further.
Once these tasks are complete, you can either wipe the area down with a wet towel or, even better, use a wet/dry vacuum to pull up any remaining dirt particles from the grout, leaving it looking clean and new. After this is done, you might want to consider applying a sealant to help prevent dirt from collecting in the future.