Even though Bitcoin is a digital currency, it can actually be stored on old-fashioned paper.
The best way to prevent being robbed of all your Bitcoins at once is to make sure that your holdings are spread out over multiple wallets in different locations.
Like any other software, the Bitcoin client is not perfect. This means that security vulnerabilities may be found, which can be a major risk for a client that is not up-to-date.
The average person will be able to remember his or her bank account number, but for a Bitcoin wallet address this is significantly harder to do.
Even though online Bitcoin wallets are convenient as they can be accessed from anywhere in the world, there are several reasons not to store large amounts of Bitcoins in them.