A strong acid is one that is 100 percent ionized in a solution, and a weak acid is one that doesn't ionize fully when dissolved in water. Sulfuric acid is an example of a strong acid, and hydrogen fluoride is a weak acid.
Strong and weak acids are not the same as concentrated and diluted acids. An acid's strength is related to how it reacts in water. The concentration of an acid merely tells how much of the acid is dissolved in a solution.
Often, a strong or weak acid can be turned into something else, which is the case with hydrogen fluoride. When dissolved in water, this weak acid produces hydrofluoric acid.