An example of online used as an adjective is in the phrase "online computer," which means a computer that is connected to the Internet.
I'll be online tonight, so I'll be able to reply to your email.
An example of online used as an adverb is in the phrase "online chat," which means chatting done over a network.
The workers' terminals are online during business hours.
Is the new factory online yet?
The power is online.
Once this factory comes online, it will double car production in our country!
An online database.
Press the F1 key to access the online help.
The program comes with an online manual.
I prefer to read the online newspapers.
He works online.