An output parameter can be used to assign a variable the result of a stored procedure. Less used now that we have user-defined functions.

For dynamic Sql, it's something like

declare @cmd varchar(8000), @a char(2)

select @a = 'US'

select @cmd = 'select gemName,gemType,State

from GEmTable where country=' + @a

exec(@cmd)

That's dynamic SQL. A regular query would be

select gemName, gemType, State from GemTable where country = @a