Xmarks is an Add-On for FireFox, Chrome, IE and Safari that allows you to use one browser on one PC or Mac, add a bookmark and then go to a different PC or Mac, yes iPad too and see the synchronized bookmarks. Now any browser you use will have all your bookmarks in sync. Also it is the best way to backup your bookmarks if you get a new system or have to rebuild your computer, the power of the cloud will store your bookmarks on the Xmarks server, which also allows you to access your bookmarks from any computer, say your friends, work or parents system. You just login to your Xmarks account and poof.. There are all your bookmarks. Xmarks synchronizes your bookmarks each time you open or close your browser or as needed. Xmarks also lets you save your passwords for logins, but they are not stored as secure as I would like them to be. The only issue with Xmarks is the lack of encryption at rest, on the Xmarks server and options to additionally secure them with two-factor authentication.
LastPass is a tool that is used to save and store all your website logins and also secure notes. Browser based Add-on as well and iDevice too, LastPass encrypts your password file locally with AES 256 encryption, so as good as it gets as well as stores them on the LastPass server, yup, the cloud, again with AES 256 encryption. So if some Ne'er-do-well breaks into the LastPass cloud servers, they would only get encrypted data that is worthless without the master password only you have.
In addition, you can add, yet another best thing, a Yubikey to add a 2nd factor 'something I have' token that you plug into your USB and touch to add a one-time and/or static password option for $25 USD that now protects your website logins and passwords with not only your username and password, but with the option of 2 factor authentication with a Yubikey, which of course I use to protect my website logins...
With LastPass acquiring Xmarks, this means we will now have a bookmark sync tool that now stores your bookmarks securely, very secure with AES 256 and with Yubikey as an option, your bookmarks, usernames, passwords and secure notes will all now be securely stored locally and in 'the cloud'.
Xmarks website
LastPass website
Yubikey website