You might have noticed that there are a number of different brake fluid types available, all classified with a "Department of Transportation" number (DOT, this is the US D.O.T , the UK is Department of Transport.
There are 2 main classes, Silicone based,(DOT 5) and Glycol based. (DOT 3/4/5.1)
Glycol based fluids will pull water out of the air, and reduce their boiling point, Silicone based fluids won't, but using Silicone in a system that has had other fluids in it can lead to water and moisture pooling in areas, which can boil and ruin your brakes.
Glycol fluids mix with the water and hold it, it will reduce the effectiveness, but it'll be gone the next time it's changed.
You're unlikely to see anything these days with DOT5 in it, likewise for DOT3 which has been replaced by DOT4.
What can be mixed?
Do not mix DOT5 with anything, DOT3, DOT4 and DOT 5.1 can be mixed with each other without problems, but it'll be as bad as the weakest link, which may be old damp fluid, or a lower rated fluid added into a better system.
What are the differences?
The important bit is these are the MINIMUM boiling points allowed to be in a certain spec.
What's the best fluid then?
If you are stuck, and have only a few options, then you'll probably find that a DOT 5.1 will be the best performing, but there are better fluids out there.
ATE Super Blue ~ £20/litre
Millers Racing Brake fluid ~ £14/500ml
Motul RBF600 ~£15/500ml
Motul RBF660 ~£20/500ml
Castrol SRF ~£40/litre
All of the above fluids are in fact DOT4s, and Castrol SRF is DOT 3/4. Castrol SRF doesn't show a boiling point in most documentation, just a statement that it's easily over 300°C, and it's Wet boiling point is massive compared with most others. It will soak up water far quicker than any other the other types, and usually is used for racing applications where changes are far more frequent.
It just goes to show that all of the best fluids are DOT4 rated, so be aware that getting a good DOT4 is always better than an average DOT 5.1
You won't feel a difference between each type, that's all down to hydraulics, but for track work, boiling the fluid will ruin your day, and potentially be quite dangerous.