For the most part LINQ works very much like SQL. Sources, joins, selects, and where clauses are all pretty standard fare. The Group/By/Into clause is where this breaks down. Unlike SQL, which always returns a rectangular dataset, LINQ is capable of returning hierarchical data.
An example of this would be grouping customers by country and city. In SQL this would require either manually grouping on the client-side or performing 1 + N + (N*M) queries: one to get the countries, one for the cities in each country, and one for each customer list.
This can all be done with a single LINQ query by using a series of sub queries. However, the query gets increasing complex. Mitsu demonstrates:
var q = from c in db.Customers group c by c.Country into g select new { g.Key, Count = g.Count(), SubGroups = from c in g group c by c.City into g2 select g2};
Demonstrating the flexibility the LINQ framework, Mitsu reduces it down this single line:
var result = customers.GroupByMany(c => c.Country, c => c.City);
Mitsu did this in a general fashion suitable for any LINQ query. You can see the source code and an explanation of how it works on his blog.