A trigger is a special type of stored procedure in SQL Server that is executed when an INSERT, UPDATE, or DELETE statement is issued against a table. Triggers are used to maintain the integrity of the data in a database by performing an action (e.g. updating another table, sending an email, etc.) when data in a certain table is changed.
A join, on the other hand, is an SQL operation used to combine data from two or more tables. The join operation allows you to retrieve data from multiple tables based on a common field or set of fields. Joins are used to retrieve data from more than one table at a time and can be used to filter the data being returned.