Some Crosstitch patterns supply both fabric (or canvas) and floss. Others only supply patterns and specifications of fabric and floss.
Aida is usually the most popular fabric (or canvas) used. It comes in various counts. A "count" is how many holes per inch. For example: 14 count Aida means that there is 14 holes per inch.
There is usually a guide to using different counts included in Pattern Only packs.
28 count is referred to as linen - and is a pain in the butt to work with but gives unbelievable results. I've only done one pattern on 28 count linen. It was a mother holding a baby for my nieces birth. A lot of concentration and good eyesight (or magnifying glasses) are needed. As with most designs interruptions are not really helpful, but with linen it's almost impossible to work on it with constant interruptions (hence I only did work on it when my 8 year old was at school or asleep).
Losing place when you are working on fabrics can be a problem, particularly with Linen. But believe me the results are defiantly worth it.
Colours are also specified in Pattern Only packs. I find that these are only a guide. You can order specific colours from most craft stores or on the internet, but if your idea of the finished product may not be the same as the manufacturers - or, like me you might have limited resources and have to work with what you have. There is usually a picture on the front of the pack and it may have be done on a light blue background, but taking into account your personal preferences, your house decorations etc you might decide an olive green would look better. Remember it's up to you how the finished product will look.