What is the meaning of 11-32 cassette and 12-25 cassette? I'm willing to buy a 2015 Shimano 105 groupset and i have got 2 options in choosing the cassette, the first is 11-32 and the other is 12-25. I have  no idea what these numbers indicate and which one would be more suitable for me knowing that i'am still a beginner.
If you imagine a cassette, will have a bunch of sprockets on it. The current 105 range (which came out last year) will have 11 sprockets, the earlier 105 had 10 sprockets.

The notation you've noticed simply means that for one of these cassettes, the smallest sprocket has 11 teeth, the largest has 32 teeth. And the second cassette has smallest sprocket 12 teeth, largest sprocket 25 teeth.

So these numbers are basically the "range" of gears covered by a cassette.

You might imagine that a large range (e.g. 11-32) is good, as it would give you lots of teeth to climb steep hills, but also a small cog which you can use to power downhill. And this is correct, but there is also a flip-side. A larger range of sprockets means that there can sometimes be a jump of a few teeth between each sprocket, which can be less smooth when changing gear. Hence, if you don't actually need the large range, a smaller range would give you a smoother ride.

As regards which one you should choose, it's not really down to whether you're a beginner or not, more to do with how good a climber you are. If you feel you might struggle going uphill, a 32-tooth sprocket is probably going to be a good choice for you.

The other thing to remember is that, when you're thinking about gears, the cassette is only half of the story, the other half being the cranks. The number of chainrings, and the number of teeth on each chainring, will also affect the gear range and smoothness. But that's probably a different question...