Offline wallets (or cold storage) is used to store large amounts of btc for safe keeping and hodling. This is very useful to many btc hodlers because this type of wallet is safe (because of the reason that this wallet is not connected to the internet). On the other hand, an online wallet is used to store minimal amounts of btc, usually for spending and moving coins from one wallet to another.
Yes and wallets installed in pc, phone and such devices are also known as offline wallet, and wallets which we can acces only when there is internet are know as online wallets. There is two defenition for both offline and online wallets!
~~MZ~~
Haha sorry I haven't stressed about their difference. I've only stated their importance and their uses. Thanks for covering up for me mate ^_^
And to OP, Online wallets are wallets that are always connected to the internet and Offline wallets are wallets that aren't connected to the internet and is used to store large amounts of btc.