What is the main difference between STP and RST (Rapid Spanning-tree) Protocols?

The main difference between Rapid Spanning Tree Protocol  and Spanning Tree Protocol  is that Rapid Spanning Tree Protocol  assumes the three Spanning Tree Protocol (STP) ports states Listening, Blocking, and Disabled are same (these states  do not forward Ethernet frames and they do not learn MAC addresses). Hence Rapid Spanning Tree Protocol   places them all into a new called Discarding state. Learning and forwarding ports remain more or less the same.

• The main difference between Rapid Spanning Tree Protocol (RSTP IEEE.1W) and Spanning Tree Protocol (STP IEEE.1D) is that Rapid Spanning Tree Protocol (RSTP IEEE.1W) assumes the three Spanning Tree Protocol (STP) ports states Listening, Blocking, and Disabled are same (these states  do not forward Ethernet frames and they do not learn MAC addresses). Hence Rapid Spanning Tree Protocol (RSTP IEEE.1W) places them all into a new called Discarding state. Learning and forwarding ports remain more or less the same.

• In Spanning Tree Protocol (STP IEEE.1D), bridges would only send out a BPDU when they received one on their Root Port. They only forward BPDUs that are generated by the Root Switch (Root Bridge). Rapid Spanning Tree Protocol (RSTP IEEE.1W) enabled switches send out BPDUs every hello time, containing current information.

•Spanning Tree Protocol (STP IEEE.1D) includes two port types; STP Root Port and Designated Port. Rapid Spanning Tree Protocol (RSTP IEEE.1W) includes two additional port types called as alternate ports and backup ports.

An alternate port is a port that has an alternative path or paths to the Root Switch (Root Bridge) but is currently in a discarding state (can be considered as an additional unused Root Port).  A backup port is a port on a network segment that could be used to reach the root switch, but there is already an active STP Designated Port for the segment (can be considered as an additional unused designated port).

Check below

https://cciethebeginning.wordpress.com/2008/11/20/differences-between-stp-and-rstp/

Simple and short: RSTP is faster and better than STP

Apart from operational differences, main difference is convergence times. STP can take as many as seconds for the loop free topology to converge, which is huge for time critical applications, while RSTP's convergence times can be in milliseconds.  

What happens to the BPDU behavior?