What are they ?

What are they used for

An alkali is a compound that dissolves to give a solution with a pH higher than 7 .Alkali refers to the Arabic word "al qaliy" or calcined ashes that provided the early source of the alkaline substance.Whether used in the construction of the pyramids in Egypt or for neutralizing the pH level of lakes fed by acid rain, alkali's are a vital part of products and substances used daily.They are used for many things in the modern society some of these things are ;

Making glass
Neutralising acidic soils
Making chemicals that bind natural dyes to cloth

Making soap

Indigestion tablets

And many more things.

There are a huge number of reactions that you can perform with alkalis and other substances.

Alkaline hydroxide + acid → salt + water
Alkaline carbonate + acid → salt + water + carbon dioxide

Also interestingly alkali's can conduct electricity due to the presence of mobile ions.