Full Idea
The is of 'Socrates is human' expresses the relation of subject and predicate; the is of 'Socrates is a man' expresses identity. It is a disgrace to the human race that it employs the same word 'is' for these entirely different ideas.
Gist of Idea
'Socrates is human' expresses predication, and 'Socrates is a man' expresses identity
Source
Bertrand Russell (Introduction to Mathematical Philosophy [1919], XVI)
Book Reference
Russell,Bertrand: 'Introduction to Mathematical Philosophy' [George Allen and Unwin 1975], p.172
A Reaction
Does the second one express identity? It sounds more like membership to me. 'Socrates is the guy with the hemlock' is more like identity.