Tile grout is required to fill the tile gaps or joints after the tile laying process.
This is to ensure waterproofing *, hygiene, tile protection and good aesthetics.
There are some exceptions where grouting is not necessary, for example, when the tiles are butt jointed for usage as a picture or piece of art as used by tile collectors.
Before applying grout, make sure to clean all adhesive the tile joints or the adhesive colour can "bleed" through the grout. Ideally you would clean the joints while installing the tiles. It is best to leave the depth of the tile joint free from adhesive in order to have full grout coverage and in turn stronger grout adhesion.
Most of considerations to take on choosing the right grout for your project, are as listed below but you should take into consideration other possible variables.
Interior, exterior, dry, wet, intermittently wet.
Wall or floor.
Joint size and depth.
Surface temperature on application and on final surface usage.
Tile resistance to scratching.
Cement based, acrylic, polymer modified, epoxy or rubber based...
Weight load and Compression.
For example, it is common to use a standard water resistant cement based grout for areas that are not subjected to noticeable movement but if for instance you have a tiled wooden section, then it is more appropriate to be using a grout with built in plasticiser or to purchase the plasticiser separately and add it.
Tip: Make sure not to mix products from different makers / brands as these could have undesired effects. Do not use PVA as an additive to grouts.
Pay attention to the manufacture's instructions on mixing grouts.
Do not mix if room temperature is below 5 °C or in damp conditions and always make sure to start grouting once the adhesive has dried out or you could risk problems with capillary fracture of the tile joints and with that, water infiltration.
Some grouts require to be mixed with water, some with resin hardener and some with its own chemical.
Where total waterproofing is required, the use of epoxy grout is recommended.
* Although some grout tubs show the word "Waterproof", you might find that in the small print, the manufacture will mention that the grout is "Water Resistant". What they mean is, that the grout is not affected by the water but some water dampness / penetration can go through the Tile joints. This is the reason for recommending the usage of "Tanking Products".
Epoxy grouts are not the first choice for many as these grouts are more difficult to apply correctly, requiring good technique, skill and speed. The usage of these type of grouts can produce a considerable amount of mess when cleaning the tiles so it is of high importance to make sure that grout-free areas and surfaces are well protected with waterproof sheets.
The colour choices are many. As a rule of thumb, a similar colour grout to the tile is often chosen but in most cases, when a floor is being grouted, the most used colour is grey as the joint can often stain after few months of walking on it, although there are some new products that can protect some grouts; Grout Sealers.
Be careful with strong colour grout as these WILL stain the tiles and surrounds. Always test a small area first. Products such as Tile Sealers are also available and these can reduce the risk of tile, stone and marble from staining.
Tile and Grout:
There are tile adhesives that can be used also as grout, like the Bal Tile and Grout. This type of grout is not as easy to apply and clean as the cement based grouts but it is great to use for when thin glass mosaic tiles are installed as there is no problem with the adhesive contamination and bleeding through.