Acids mainly have H+ ions in their formulas. When ionized in water, acids yield H+ ions into the water.
Bases mainly have OH- ions in their formulas. When ionized in water, bases yield OH- ions into the water.
Based on this, we can conclude that acids have higher concentrations of H+ while bases have higher concentrations of OH-.
If a solution has equal concentrations of H+ and OH-, then this solution is said to be neutral.