There has been a lot been discussed about SDF and TFT I would suggest to go through the http://tech.queryhome.com/14082/what-is-the-relation-between-sdf-and-tft-at-pgw-pcef
In short:
Service Data Flow is mainly used for bearer association for the incoming IP packets from the external data network. Each IP packet has "Type of Service" field which helps router to schedule IP packets differently since every service requires different treatment for its packets. SDF does this job and send an IP packet to an appropriate bearer. Where as Traffic Flow Template is per bearer, It is nothing but collection of packet filters.
