In simple terms the design of the chip is different thus allowing faster processing speeds. The 486 series was the hit of the late 90's but after that, Pentium 75s and 255s came out replacing the slow 486. With faster processing other technology could be applied such as MMX. Chips these days go up to 4Ghz and more, but are not necessarily Pentiums.
All of what JB has said is true, grouptt - generally speaking a Pentium processor, especially when coupled with a lot of memory, means performance and speed for a computer, especially when compared to a 486 processor from 10 years ago. In terms of perfomance and ability, a 10-year old 486 machine is a pocket calculator compared to today's high-end Pentium processors.
An easy way to tell if a modern computer has a Pentium processor is to check the base unit casing. Most modern Pentium-equipped PCs have a decal on the front that reads "Intel Inside - Pentium"
Pretty much all modern PCs these days have either Intel Pentium 4, Intel Celeron, or AMD Athlon processors, and most will have labelling on the front of the casing which will tell you.
Another way to see which processor your system has is to check System Properties in Windows. To do that, right-click the My Computer Icon and choose Properties from the menu. When the window opens, it will tell you in the lower segment what type of processor you have and how many megabytes of system memory (RAM).