Concentrated vs. Dilute; Strong vs. Weak
These terms are often the most misused in chemistry.  Concentrated and dilute refer to the concentration of  an acidic or basic substance in a solvent.  eg. 16 M HCl is more concentrated than a 0.5 M solution of the same acid.
Strong and weak refer to the ability of an acid or base to dissociate. A strong acid will dissociate completely in water to form hydronium ions. i.e. 100% of it will form H3O+.   A weak acid or base will only dissociate to a certain percentage. Often a very small percentage only.
Strong and Weak Bases
NH3(aq) is a poor conductor of electricity when compared with NaOH(aq). This means that the degree of dissociation of NH3  in water is relatively small when compared with that of the NaOH.  A base which is only slightly dissociated in aqueous solution is called a weak base; one which is highly dissociated is called a strong base.  All the strong bases happen to be inorganic, that is, the NaOH, KOH, RbOH group. Even Ca(OH)2 and Ba(OH)2 are considered to be strong bases. All of the rest are too insoluble to provide a significant [OH-] in water.  The double arrow convention should be used when dealing with a weak base. A single arrow is to be used when showing the dissociation of a strong base since for all practical purposes, dissociation is 100% complete. i.e., a water solution made from NaOH(s) will have no molecules of NaOH in it.  The NaOH will be completely ionized into Na+ and OH-.