Difference between service pack and Patch in XI

What is the difference between Service Pack(SP) AND Patch ??

e.g SP12, Patch8.

Is it like that when there is a major change they will issue SP and

for a small change they will issue patch ?? or after number of patches

they will issue SP combining all the patches ??

Service Pack level can be seen from SAP gui system..>status.

But Patch level where can we see ?? what is the Tcode for that ?? or just we got

to go through below links ??

http://<host>:<j2ee_http_port>/sap/monitoring/ComponentInfo

also

http://<host>:<j2ee_http_port>/sap/monitoring/SystemInfo for core system info (J2EE or JVM patch level, etc.)

thanks

kumar