Alkalis produce OH⁻ ions in aqueous solution.
NaOH  →  Na⁺ + OH⁻

Alkaline solutions have pH values more than 7. The higher the pH, the stronger the alkali.

Strong Alkali
A strong alkali is one, which can completely dissociate its OH⁻ ions in water. This means that it can give off the greatest number of hydroxide ions when placed in solution.
e.g. NaOH, Ca(OH)₂

Weak Alkali
A weak alkali is one, which does not completely dissociate its OH⁻ ions in water. This means that it does not give off many hydroxide ions when placed in solution.
e.g. Ammonia (NH₃)

ALKALI REACTIONS

Alkali + Acid
Acids react with alkali to form a salt and water.
Acid + Alkali  →  Salt + Water
E.g. HCl + NaOH -> NaCl + H₂O
This is a neutralisation reaction.