what is difference between ewallet and acount in my PC/USB?

[predic]

Hi, I am new to bitcoins and I am interested for difference between my bitcoin account in my PC (I use PCs in libraries) and ewallet (bit-bank.org for example)?
can I use bitcoins without to download bitcoin software? for example I use bit-bank.org and people can send me BTC and I can send them, I can even use bit-bank API for online shop if I make my e-shop, so, I am asking myself, why to install software, even PC is not mine and I don't know if I can use USB (I have USB with password login), when I can use bit-bank? and where to check if anyone had bad experience with bit-bank? many website are new and I don't think people register company for website, they just make website and take some small provision and if someone take your bitcoins, he can simply close website and disappear.
so, main question is, can I participate in market of Bitcoins without to install software?

[dogisland]

Basically counterparty risk. http://en.wikipedia.org/wiki/Credit_risk#Counterparty_risk

You have to trust that bit-bank won't steal your coins or that their security is good enough that no one can hack their servers and steal your coins.

Whereas if you keep you coins in your own wallet you run the risk of hackers, trojans and wallet loss. But you'll be OK for counterparty risk.

[predic]

Yeah, i understand, but what is with people who use public libraries, is it possible to install Bitcoin software on USB?

[Stephen Gornick]

Yeah, i understand, but what is with people who use public libraries, is it possible to install Bitcoin software on USB?

Are you asking about a portable app that can be run from USB?

I had seen a thread on one but since the source wasn't being released I personally wouldn't use it.

Running it as a portable app doesn't lessen the security risk all that much though.

 - http://en.bitcoin.it/wiki/Securing_your_wallet