In LINQ queries, the 'join' keyword is employed to join a sequence of data together. The sequence can be any collection type, such as a list, array, or dictionary. The 'join' keyword takes two arguments: the sequence to be joined to and the join predicate (a Boolean expression). The join predicate specifies which elements of the first sequence are to be joined to those of the second sequence.

The join statement returns a sequence that includes the elements of both sequences that meet the join condition. For instance, the LINQ query below merges two lists of learners and courses.

var students = new List<Student> {
   new Student { Name = "John Doe", Course = "Math" },
   new Student { Name = "Jane Doe", Course = "English" },
};
var courses = new List<Course> {
   new Course { Name = "Math" },
   new Course { Name = "English" },
   new Course { Name = "Science" },
};
var joinedStudents = students.Join(courses,
   (student, course) => student.Course == course.Name);

This query returns a list of students, with each student's name and the course in which they are enrolled. The 'join' keyword is used to perform various kinds of joins, including inner and outer joins, as well as self joins, according to the join predicate.

Here are some examples of different types of joins:

Inner join: An inner join returns a new sequence that contains the elements from both sequences that satisfy the join predicate.
Outer join: An outer join returns a new sequence that contains all of the elements from the first sequence, even if they do not have a match in the second sequence.
Self join: A self join joins a sequence to itself. This can be used to find relationships between elements within the same sequence.

The `join` keyword is a powerful tool that can be used to perform complex queries on data. It is a valuable addition to the LINQ query syntax.