RTP Encryption, SDES, AES

media_encryption=sdes

I have an understanding problem.
What does the “S” at SDES stand for?

Doesn’t this “DES” have anything to do with the Data Encryption Standard?

Wikipedia says: SDES (Session Description Protocol Security Descriptions)
Google search shows also: Simplified Data Encryption Standard (SDES)

Because TLS and RTP encrypt with AES right?

Wikipedia is right. It is about the way the encryption keys are negotiated via SDP. The draft of the RFC 4568 was called SDescriptions and this was further abbreviated and is called SDES-sRTP now. It is in contrast to DTLS-sRTP and zRTP(-sRTP).