The concentrated acid are the acid which has hundred percent of acid content in it (ideally little water is always present) and the dilute acid is when you add the solvent to the acid (water) you dilute the acid. The more solvent you add more dilute the acid becomes. The precautions should be taken while working with con. acids.
Added: [More according to GLP (
Good Laboratory Practice)]
The difference is gradual, but not sharply drawn:
Diluted means (more) lower concentration, in laboratory practice less than 1.0 M (<1.0 mol/L) and concentrated acids (or other solutions) are higher concentrated, above 1.0 mol/L .