Pure water can dissociate to form H+ and OH- ions. This is a reversible process and can occur in both the direction. H+ refers to the hydrogen ions, whereas OH- refers to the hydroxide ions.
Acids are the compounds that on dissociation in a solution, produces H+ ions. Acids have more of H+ ions and lower pH than 7. Strong acid like hydrochloric acids produce more H+ ion concentration as compared to the weak acids like acetic acid.
Bases/alkalis are the compounds that on dissociation in solution, produces OH- ions. These OH- ions have higher pH value than 7. Strong bases like sodium hydroxide produce more concentration of OH- ions than weak bases/alkalis.