Tyre replacement for Rockrider 300 I have a rockrider 300 which has tyre measurements as 26*2.0 and tube measurements as 26*1.75-26*2.12. According to the catalog they are semi-slick tyres. I want them to be even slicker. 

I have looked up a btwin tyre online which measures 700*25 which is compatible with tubes measuring 700*18 - 700*25 and 700cc or 28 wheels. I am not sure what any of this means and if its a good for my bike in terms of measurements. 

Can someone please give me a range of measurements of tyres and tubes that I can buy which can fit my rim.
Ah the wonderful world of tire sizes, where nothing is what it seems! There are multiple size systems (French, fractional, decimal, ISO), and only ISO/ETRTO, the "international standard" is (somewhat) consistent. Unfortunately, many people don't use the ISO/ETRTO system.

The definitive guide to tire sizes is Sheldon Browns' site:
http://sheldonbrown.com/tire-sizing.html
The only thing I would like to add to the mighty Sheldon:

Avoid any tire with fractional markings (such as 26 x 1 1/4) like the plague, unless you are 100% sure what you are doing. 

On to your question:

Your current tire: 26 x 2.0

This one of the most common MTB sizes, measured in inches:

26 in. diameter, 2.0 in. width.

Equivalent ISO = 599mm (diameter) x 50mm (approx. width)

Your new Btwin tire: 700 x25c

700 is a French size often used for road/racing cycles. Since the French like their millimetres, a 700 x 25 means "700" diameter, 25mm width. To complicate things, the French "700" is not the same as the ISO diameter...

Equivalent ISO  =  622mm (diameter) x 25mm (width).

Since the diameters are not the same, your new tire will not fit. There is also a large difference in tire width (50mm for the old tire, 25 for the new)

General tire selection advice:

1) Get the ISO diameter size right first. This is the only measure that can be somewhat relied upon.

2) Check tire width &amp; rim compatibility.
Get the diameter size right first, then check the tire width. Small/skinny tires will not fit on very wide MTB rims. Fat tires will not fit on narrow rims. Measure your rim width. Sheldon Brown's site has a table that shows rim width vs. tire width compatibility. 

From personal experience, I have established these quick and dirty rules regarding tire width:

Racing cycle wheels: 23mm - 30mm (but the wider tires probably will rub the frame)

City bike wheels: 30mm - 47mm 

MTB wheels: 35mm- 60mm