It stands for Web Template Framework, but it's funny it appears all the time in crash reports:

http://google.com/search?q=wtf+webkit+crash

For example:

    Thread 2 name:  WebThread
    Thread 2 Crashed:
    0   JavaScriptCore                  0x32a97d46 WTF::StringImpl::hash() const + 42
    1   JavaScriptCore                  0x32a97c6a WTF::HashTable<WTF::StringImpl*, WTF::StringImpl*, WTF::IdentityExtractor<WTF::StringImpl*>, WTF::StringHash, WTF::HashTraits<WTF::StringImpl*>, WTF::HashTraits<WTF::StringImpl*> >::lookupForWriting(WTF::StringImpl* const&) + 18
    2   JavaScriptCore                  0x32a9b028 std::pair<WTF::HashTableIterator<WTF::StringImpl*, WTF::StringImpl*, WTF::IdentityExtractor<WTF::StringImpl*>, WTF::StringHash, WTF::HashTraits<WTF::StringImpl*>, WTF::HashTraits<WTF::StringImpl*> >, bool> WTF::HashTable<WTF::StringImpl*, WTF::StringImpl*, WTF::IdentityExtractor<WTF::StringImpl*>, WTF::StringHash, WTF::HashTraits<WTF::StringImpl*>, WTF::HashTraits<WTF::StringImpl*> >::add<WTF::StringImpl*, WTF::StringImpl*, WTF::IdentityHashTranslator<WTF::StringImpl*, WTF::StringImpl*, WTF::StringHash> >(WTF::StringImpl* const&, WTF::StringImpl* const&) + 448
    3   JavaScriptCore                  0x32a9ae30 WTF::AtomicString::addSlowCase(WTF::StringImpl*) + 96
    4   WebCore                         0x31bade0c WebCore::AtomicHTMLToken::initializeAttributes(WTF::Vector<WebCore::HTMLToken::Attribute, 10ul> const&) + 232