The strength is a measure of the degree of ionization of a substance in an aqueous solution.
Some substances are strong and they dissociate completely (this is the case of hydrochloric acid, HCl, in water), some are weak and they do not dissociate completely (like acetic acid, CH3COOH).
The concentration is a measure of the amount of the substance in a solution and it is usually explessed in moles or percentage. It is a ratio of the substance to the solvent. A concentrated solution is a solution with a high percentage of the substance (solute) in the solution, and a dilute one is a solution with a low percentage of solute.