When you type a function in Matlab, quickly a suggestion box comes up showing functions similar to the one you're typing. I thought that Mathematica didn't have this but one day, I typed half of a function and the went to the kitchen when I got back, the suggestion box was there. I'm not sure if it appears after some time or if I accidentaly pressed some key.
Do you mean CtrlShiftK?
As pointed by Yves，CtrlK will also work，but CtrlShiftK will work differently if you finish the function name. For an example,
Mathematica will show:
Besides the nice real handy option suggested by @yulinlinyu (more here) you can also use text-based interface to find completion for your half-typed function. It is not that fast, but has its own advantages. Try executing this:
and you'll get this nice table of possible functions that complete your input. If you click on any you'll get short definition - as you can see below. And if you click on
This is how, for example, I discovered that Mathematica supports the awesome
Sometimes instead of handy shorthands like in the other answers, you'll find more useful
If you want to look at the documentation of a given name :
To count the number of names let's define :