These are new tags intorduced in Flex version-4.
<s--stands for spark componets that are introduced newly in Flex4.
<mx--mxml components that are used in Flex 3 and also supported in Flex4
As Paul suggested we have to use proper namespaces in order to use them.
For more information look for Adobe documentation site.
subject: what is the difference between <s,<mx,<fx tabs