http://en.wikipedia.org/wiki/List_of_Intel_Core_i5_microprocessors#.22Sandy_Bridge.22_.2832_nm.29_2
Models and differences over standard 2500 in bold:
Core i5-2500: 4 cores, 3.3Ghz (Turbo 1/2/3/4), HD Graphics 2000 (850–1100 MHz) - 95W TDP
Core i5-2500K: 4 cores, 3.3Ghz (Turbo 1/2/3/4),
HD Graphics 3000 (850–1100 MHz),
unlocked multipliers - 95W TDP
Core i5-2500S : 4 cores,
2.7 GHz (Turbo 1/5/9/10), HD Graphics 2000 (850–1100 MHz) -
65W TDP -- (low power)
Core i5-2500T: 4 cores,
2.3 GHz (Turbo 1/5/9/10), HD Graphics 2000
(650–1250 MHz) -
45W TDP -- (ultra-low power)
HD Graphics 3000 > HD Graphics 2000
TDP = Maximum thermal output (how much heat the CPU outputs and also correlates to power usage, eg. a 95W CPU uses at LEAST 95W, probably more, at load that is).
Turbo = How much the CPU increases clock speed depending on load. Guide:
http://en.wikipedia.org/wiki/Intel_Turbo_Boost#Example