What's the role of "Key" and "Applications" ?

[Alwaysin]

Hello,

I'm new to Zabbix and for the moment, there are two things I didn't understand :
- What is the usefulness of the "key" (when you create a new item)
- and what is the usefulness of Applications

I searched in the manual and the forums, but didn't find out any answers to my questions.

Thanks in advance.