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...
Numbers denote number of teeth on smallest cog i.e. 11-32 has smallest cog with 11 teeth, and largest with 32 teeth.
11-32 gives you larger range of transmission ratios while 12-25 gives you smoother steps between the speeds. When calculating which one to choose also take into account number of teeth you have in front, and the kind of terrain you would like to drive. Also make sure your rear derailleur has capacity to consume chain slack for your given combination
Smoother steps between the gears is a nice to have - but if the gearing is not suitable to you - it becomes irrelevant.
It's mentioned in the answer above "rear derailleur has capacity". This refers to the the length of the cage on the rear derrailleur. They come in three varieties (with Shimano anyway) - short, medium and long. Each length has a specified teeth capacity.
The teeth capacity required is calculated as the total difference between smallest and largest sprockets on both chainset and cassette. ie. 50/34 with a 28/11 is 33t - check with the manufacturer but iirc this is a medium cage.
(to complicate things - you can always squeeze a few extra teeth beyond the manufacturer's suggestion).
The number of "speeds" or count of cogs is also an important number, which hasn't been stated in your initial question.
A bike set up with N speeds or "gears in the rear cassette" can have the cassette swapped for others with different tooth counts, BUT you're stuck with the number of speeds.
In other words, a 9 speed bike has 9 distinct cogs in the back; you can see and count them. A replacement cassette MUST have the same number of gears, otherwise you start down the slippery slope of knock-on upgrades, and have to change your shifter lever and chain, then perhaps the derailleur. And the cables may as well be changed too.
So properly-speaking a cassette is described as "11-32, 10 speed" or "12-25, 8 speed"