An argument is valid when it contains no logical fallacies. Such arguments are not necessarily sound, because the premises may be false. For instance, the following syllogism is valid but has a false conclusion:

All animals are dogs. (False premise/All A are B.)
All dogs are terriers. (False premise/All B are C.)
Thereforse, all animals are terriers. (False conclusion: not all animals are terriers. Valid logic: If all A are B, and all B are C, then all A are C.)

Blog posts

The Parable of Hemlock (tangentially related)

See also

Sound argument