What are collective responsibility and Individual Ministerial Responsibility? and how are they different?

Collective Responsibility applies to all Ministers and is designed to ensure government unity - all ministers have to follow cabinet decisions. Therefore, if a minister cannot abide by a cabinet decision they must resign; eg, Robin Cook MP over the Iraq War. There are some exceptions in cases where there have been agreements to differ or free votes as in the case of the AV and EU referenda.

Individual Ministerial Responsibility is when a Minister resigns in order to take responsibility for the actions of their department or for their own personal actions. This convention has been undermined through Civil Servants beng fired instead - normally a Minster only resigns in this instance when there is particuular political and media pressure and increasingly it is due to personal conduct rather than taking responsibility for a failure with regards to their department. Examples of this type fo resignation include Andrew Mitchell - plebgate (Oct 2012), Chris Huhne - speeding ticket case (Feb 2012).

Therefore, whilst both are instances of Ministers resigning, there are key differences for the circumstances and reasoning for which they are used. They are both conventions based on the Ministerial Code of Conduct. However, as we saw especially during the coalition years, these conventions can and have been undermined during the coalition years.