HSRP vs. SRST

Could someone clarify the major differences between HSRP and SRST? When would a person use one over the other? (I have my own notes, but I want to see what others say).
"Computers in the future may weigh no more than 1.5 tons." - Popular Mechanics, 1949
Comments
SRST is related to telephony, when the wan link goes does, calls are routed through the PSTN
HSRP, relates to having a backup gateway. So if a gateway goes down the secondary kicks in, and the end user see's no outage (assuming you configure it correctly)
Thanks for the clarification.