What is the Difference between Strong and Weak Alkalis?

Alkalis and acids can be described as strong or weak.
This does not mean the same as concentrated or dilute.

The strength of an alkali (or acid) depends
on how ionised it is in water.
An alkali forms hydroxide ions (OH^-  ions) in water.

What is a Strong Alkali?

A strong alkali is completely (100%) ionised.
An example of a strong alkali is sodium hydroxide.
A strong alkali has a pH of 14.

sodium hydroxide (in water)      sodium ion  +  hydroxide ion
NaOH_(aq)                                   Na⁺_(aq)    +        OH^-_(aq)

Sodium hydroxide exists only as ions both as a solid and
dissolved in water (see examples of other strong alkalis).

What is a Weak Alkali?

A weak alkali is only partly (less than 100%) ionised.
An example of a weak alkali is ammonia.
A weak alkali has a pH of 11 or 12.

ammonia   +   water    ammonium ion  +  hydroxide ion
NH_3(g)  +      H₂O_(l)             NH₄⁺_(aq)    +      OH^-_(aq)