What is ISR router? What is the difference between ISR and normal router?

An ISR will include the smarts and connections to support telephony (Voice over IP) and a number of other specialist components such as Intrusion Prevention. Cisco coined the phrase to differentiate their products from other router vendors (who now also include many of those features).

 Presumably these days, a non-ISR router would just route packets between different networks (which is the original definition of them)

Based on the performance and features and capabilities, Cisco offers3 major routers types:

1.       Cisco ISR: (integrated Service Routers):  typically used as internet edge for customers with small/medium networks.

2.       Cisco ASR (Aggregation Services Routers): typically used as internet edge for enterprise networks and service providers.

3.       Cisco CSR (Carrier Routing System): typically used in (IP/MPLS) core networks and for high-speed peering in smaller point-of-presence (POP) locations.

ISR is for Branch Operation.There are4451-X,800 Series,1900 Series,2900 Series,3900 Series routers.Please check below ISR Series routers features:

http://www.cisco.com/en/US/products/ps10536/prod_series_comparison.html

ASR is for Network Edge or Service Providers.There are ASR903 Series,XR12000/12000 Series,7600 Series,1000 Series,9000 Series for Service providers. ASR1000 series for Network Edge.

Hope this link could help you much:

http://www.cisco.com/en/US/products/hw/routers/index.html

As amentioned by Osama, ISR/ASR?CSR routers are routers that normally used at network edges, capable of adapting various types data streams like voice, video as well as data, with different levels of data protection features..