Full Idea
The Rule of Detachment is a version of Modus Ponens, and says 'If |=φ and |=φ→ψ then |=ψ'. This has no assumptions. Modus Ponens is the more general rule that 'If Γ|=φ and Γ|=φ→ψ then Γ|=ψ'.
Clarification
'Γ' names a set of assumptions
Gist of Idea
MPP: 'If Γ|=φ and Γ|=φ→ψ then Γ|=ψ' (omit Γs for Detachment)
Source
David Bostock (Intermediate Logic [1997], 5.3)
Book Reference
Bostock,David: 'Intermediate Logic' [OUP 1997], p.202
A Reaction
Modus Ponens is actually designed for use in proof based on assumptions (which isn't always the case). In Detachment the formulae are just valid, without dependence on assumptions to support them.