XPath (and by extension XSLT) are affected by the namespaces present in the document you are traversing/transforming as described nicely by Dare here. It is possible to write XPath expressions that effectively ignore namespaces however by using the local-name() XPath function so in a sample document like this
<foo xmlns='urn:foo'>
<bar>
<asdf/>
</bar>
</foo>
This expression will match the “bar” element //*[local-name()='bar']
This one won't //bar
Of course namespaces exist for a reason, so just blithely ignoring them is not necessarily the greatest idea except in the most ad-hoc circumstances. If you don't want to use namespace prefixes in your expressions, or if the namespace you want to match elements from is the default namespace which doesn't have a prefix you can use the namespace-uri() XPath function in conjunction with local-name() to match nodes like this
This will match the “bar” element in the urn:foo namespace //*[local-name()='bar' and namespace-uri() = 'urn:foo']
This is obviously much more long-winded than writing //f:bar if you've aliased f to urn:foo, but it can sometimes come in handy.