A distinction can be made with regards you are going to upload your personal files to the online storageservers. Some services offer a webinterface to upload your files. You are logging in with your username and password and then you can select within the browserinterface the files you want to upload for backup. This solutions works fine in case you are working with several computers and you want to be flexible regarding uploading your files to backup service.
Other online storage companies work with client, this means that you need to download software. You tell the software which files, folders or filetypes (for example all jpg or mp3) needs to uploaded automatically. In case you are working with the same pc all the time, it is a good solution. You can instruct the client to upload any file in one or more specific folder. In case you add or mutate a file in these specific folder(s), the files will be backupped automatically. Mozy is an example of an excellent free backup service that works like this.