Grout is used to fill the gaps in-between ceramic and natural stone tiles. The most common grout is a cementitious based powder that is mixed with water to produce a thick slurry that is spread over the tiles with a grout float and worked into the gaps. Excess if wiped off with a sponge and again when the grout has dried.
Grout is available in several colours which can be used to blend in or contrast with your chosen tiles. White is the most popular colour for wall tiles but this is the most difficult colour to keep clean. Grout colours can also be used to delineate between wall and floor tiles as seen in the following image:
Here, the wall tiles are a very light grey and have been grouted with a dark grey grout. The floor tiles are a dark grey and have been finished with light grey grout to give the effect of the floor being a "negative" of the walls.
The popularity of tiles has resulted in a marked increase in the number of customers fitting tiles over wooden floors. New flexible adhesives and grouts have been formulated to help with the process to and to attempt to eliminate the expansion and contraction issues which affect this type of installation. The grouts have additives in the mixture to try and prevent the grout cracking after use (wood expands a lot more than tiles and the differences in movement can crack cementitious grout). Underfloor heating can produce similar problems even when used on a solid masonry floor.
The polymers added to grouts can make them more flexible but they can have downsides, especially when used with natural stone. The grout can stain the tile, especially if the stone is not sealed. It is best to test the grout on a spare tile to ensure it will not cause problems. When the grout is applied it needs to be spread over the whole tile prior to being wiped off, not just up and down the joint lines as this can leave visible tram lines limited to where the grout was applied. Coarse grouts can cause scratches on the surface of the tiles, so ensure that the material has is finely textured.
The width of the grout joint will depend on the spacers used and the look you are trying to achieve. Most grouts will state the maximum width and depth for which it is suitable. Careful application is needed with wide joints as it is very easy to scoop out grout when wiping off excess. It is essential that the adhesive if fully set before the grout is applied. Large tiles with small gaps leave very little room for the drying out process and will take longer than small tiles with large gaps.
Tile grout can be problematic in bathrooms and showers. Many cementitious grouts are not waterproof (neither are many adhesives). Using these will result in grout absorbing water and remaining wet which will then lead to mould growth. More serious problems can arise if this is left needed as water can get behind the tiles, damage the wall and structure of the property, destroy the adhesion of the fixing compound and consequently the tiles will fall off. There are grout sealers available and also grouts that contain fungicide which can help remedy some of these problems. There are also alternatives to tiles that have no grout. For walls there are waterproof wall panel systems while vinyl flooring is available that can provide a seamless, waterproof finish.