Each series of routers will have a general data sheet to describe different model features within that series. A series is defined by a major model number, such as 1900 series, 2900 series, 3900 series, etc. Here are some examples of documents that you find by looking on Cisco.com under Routers:
Here is a sheet detailing the ISR G2 platform in general: http://www.cisco.com/en/US/prod/collateral/routers/ps10538/aag_c45_556315.pdf
Sheet with detailed module support on the ISR G2 platform: http://www.cisco.com/en/US/prod/collateral/routers/ps10538/aag_c07_563807.pdf
1900 series data sheet: http://www.cisco.com/en/US/prod/collateral/routers/ps10538/data_sheet_c78_556319.html
It's a lot of reading, and that is only a small sample. Here is a hard copy that tries to keep up to date about most of their major products: https://cisco.mediuscorp.com/market/cisco/productView.se.work?/nxt/rcrs/proieidentity/=17787
It's not as up to date as their websites, but it is a good guide to give you the overview for devices to point you in the right direction. Great as a quick reference guide, which is it's primary goal.
Half the difficulty about hardware is knowing all of their capabilities and when to deploy what kind of hardware. I spend hours each week reading about new hardware platforms and I still am far from an expert on Cisco hardware. It's a constant process.