Glass Fusing
Glass fusing is the bonding of individual pieces of glass using a kiln.
Elsewhere (see links) you can find much greater detail as regards the particular temperatures that the kiln is fired at for different effects and there are also other aspects that can be varied such as; how long it stays at the required temperature and the curve against time of how it heats up and cools down.
As with most practical pursuits, there are lots of hazards. Frit for example is finely crushed glass and is often spread by filling and then tapping small sieves raised above the target - even the slightest breeze can cause a drift so I do use a full mask. Even the simple cutting of glass comes with dangers beyond the inevitable cuts - in particular you often find that small slivers of glass leap up as you score and the eyes are very vulnerable - again I always use eye protection just in case.
You do need a great deal of expensive kit and rather than invest in such I attend workshops at the University of Hertfordshire - around £20 per night. You can buy small personal kilns but the size and variety of kit available in the university makes for me the best option. I also like working in the company of others - you can learn a great deal by talking with and observing other people - seeing their sucesses and failures as well as sharing your own.
Commercial sites that offers a number of instructive videos and reference documents are:
https://www.warm-glass.co.uk/accessory-glass-glass-kits-c-98_55.html
http://www.bullseyeglass.com/education/fusing-basics.html
Others can be found on the web and particularly on Youtube
Types of Glass for fusing
There are a number of different types of glass however I have only used two - float glass and bullseye.
Float glass was invented fairly recently during WW2 by Pilkingtons. Prior to this, sheet glass was made by pouring liquid glass onto a flat surface and then rolling it flat.
Float glass however is made by pouring molten glass onto a bed of molten tin. Float glass is also called soda or window glass. This type of glass is relatively cheap and accessible. I get most of mine from skips outside local glaziers (I ask them beforehand of course). It is not advisable to use old float glass for example from an old greenhouse as it may well have accumulated impurities that spoil the finish.
Bullseye glass is much more expensive however you can buy a very wide range of colours and types. The finish is typically much better than float glass.
It is not possible to mix different types of glass in one project because they have different expansion coefficients.