equi join ,
non-equi join,
self join,
outer join,
cross join.

There are 2 main type of joins :
Equijoin : Join N number of tables for equality =.
Non-Equijoin : Join N number of tables for other than
equality such as <>,<,>,IN,BETWEEN etc.

Technically there are total 3 different types of joins:
Cross Join: When join condition is missing then all rows
from one table gets joined with all rows from other table..
Inner Join: If a row has a NULL value in any one of the
columna of join conditin then that row will not fetched.
Outer Join: Even if the NULL values are present in any
columns of join condition that record will get fetched.
There are again 3 types of outer joins: Left, Right, Full.
Self Join : A table is joined with self.

Joins in Oracle 10g :
---------------------

A join is a query that combines rows from two or more 
tables, views, or materialized views ("snapshots"). Oracle 
performs a join whenever multiple tables appear in the 
query's FROM clause. The query's select list can select any 
columns from any of these tables. If any two of these 
tables have a column name in common, you must qualify all 
references to these columns throughout the query with table 
names to avoid ambiguity.

Join Conditions
---------------
Most join queries contain WHERE clause conditions that 
compare two columns, each from a different table. Such a 
condition is called a join condition. To execute a join, 
Oracle combines pairs of rows, each containing one row from 
each table, for which the join condition evaluates to TRUE. 
The columns in the join conditions need not also appear in 
the select list.

To execute a join of three or more tables, Oracle first 
joins two of the tables based on the join conditions 
comparing their columns and then joins the result to 
another table based on join conditions containing columns 
of the joined tables and the new table. Oracle continues 
this process until all tables are joined into the result. 
The optimizer determines the order in which Oracle joins 
tables based on the join conditions, indexes on the tables, 
and, in the case of the cost-based optimization approach, 
statistics for the tables.

In addition to join conditions, the WHERE clause of a join 
query can also contain other conditions that refer to 
columns of only one table. These conditions can further 
restrict the rows returned by the join query.

Equijoin / Inner Join
---------------------

An equijoin is a join with a join condition containing an 
equality operator. An equijoin combines rows that have 
equivalent values for the specified columns.

Cartesian Product
-----------------
If two tables in a join query have no join condition, 
Oracle returns their Cartesian product. Oracle combines 
each row of one table with each row of the other. A 
Cartesian product always generates many rows and is rarely 
useful. For example, the Cartesian product of two tables, 
each with 100 rows, has 10,000 rows. Always include a join 
condition unless you specifically need a Cartesian product. 
If a query joins three or more tables and you do not 
specify a join condition for a specific pair, the optimizer 
may choose a join order that avoids producing an 
intermediate Cartesian product.

Outer Join
----------

An outer join extends the result of a simple join. An outer 
join returns all rows that satisfy the join condition and 
those rows from one table for which no rows from the other 
satisfy the join condition. Such rows are not returned by a 
simple join. To write a query that performs an outer join 
of tables A and B and returns all rows from A, apply the 
outer join operator (+) to all columns of B in the join 
condition. For all rows in A that have no matching rows in 
B, Oracle returns NULL for any select list expressions 
containing columns of B. 

Outet join has Two types :
--------------------------
1. Left Outer Join
2. Right Outer Join

Self Join
---------

A self join is a join of a table to itself. This table 
appears twice in the FROM clause and is followed by table 
aliases that qualify column names in the join condition. To 
perform a self join, Oracle combines and returns rows of 
the table that satisfy the join condition.