Oracle released Java 7 Update 21 and Java 6 Update 45 on April 13 2013. Oracle has defined this as a Critical Patch Update, because it patches dozens of security vulnerabilities that could allow for unwanted code or malware to be installed just by visiting a website with such malware.
If you have older versions of Java, you should remove them as soon as possible. The http://www.java.com/en/download/faq/remove_olderversions.xml page has more information and tools on how to remove older versions of Java.
As a additional security precaution, you can disable the execution of Java content in your browser.