The f indicates it's a floating point literal, not a double literal (which it would implicitly be otherwise.) It hasn't got a particular technical name that I know of - I tend to call it the "letter suffix" if I need to refer to it specifically, though that's somewhat arbitrary!
For instance:
float f = 3.14f; //Compiles
float f = 3.14; //Doesn't compile, because you're trying to put a double literal in a float without a cast.
You could of course do:
float f = (float)3.14;
...which accomplishes near enough the same thing, but the F is a neater, more concise way of showing it.
Why was double chosen as the default rather than float? Well, these days the memory requirements of a double over a float aren't an issue in 99% of cases, and the extra accuracy they provide is beneficial in a lot of cases - so you could argue that's the sensible default.
Note that you can explicitly show a decimal literal as a double by putting a d at the end also:
double d = 3.14d;
...but because it's a double value anyway, this has no effect. Some people might argue for it advocating it's clearer what literal type you mean, but personally I think it just clutters code (unless perhaps you have a lot of float literals hanging around and you want to emphasise that this literal is indeed meant to be a double, and the omission of the f isn't just a bug.)
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