Using WIF, what is the difference between audienceUris and realm?

We have an ASP.NET application using WIF. Our web.config file has a section like this:

<audienceUris>
    <add value="https://someapp.mycompany.com/App/" />
</audienceUris>
<federatedAuthentication>
    <wsFederation passiveRedirectEnabled="true" issuer="https://adfs.mycompany.com/adfs/ls/" realm="https://someapp.mycompany.com/App/" requireHttps="true" />
    <cookieHandler requireSsl="false" />
</federatedAuthentication>

Every example I see both the audienceUris and realm are the same value. What is the difference between these two? Do we need them both?