Which type of relationship requires an intersection table?
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Many-to-One (M:1)
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One-to-One (1:1)
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Many-to-Many (M:M)
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One-to-Many (1:M)
Many-to-Many (M:M) relationships require an intersection table (also called a bridge or junction table) to implement the relationship. In a M:M relationship, multiple records in Table A relate to multiple records in Table B - this cannot be represented with simple foreign keys. The intersection table stores the associations, containing foreign key references to both parent tables. M:1 uses a single FK in the child table, 1:1 can use FK in either table, and 1:M uses FK in the many side - none require intersection tables.
A many-to-many relationship can't be represented directly with a simple foreign key, because each row on either side can relate to many rows on the other side. Relational databases resolve this by introducing an intersection (junction) table that holds pairs of foreign keys, effectively breaking the M:M relationship into two one-to-many relationships. One-to-one and one-to-many/many-to-one relationships can be modeled with a foreign key alone, so they don't need this extra table.