One-to-one relation
One record of the first table will be linked to zero or one record of another table. For example, each employee in the Employee table will have a corresponding row in EmployeeDetails table.
Employee EmployeeDetails
EmployeeID EmployeeID
Name,... Detail,...
Each employee has none or just one corresponding record in EmployeeDetails.
One-to-many relation
One-to-many is the most commonly used relationship among tables. A single record from one table can be linked to zero or more rows in another table.
For example, each employee has zero or more records of address.
The Employee and Address tables are linked by the key column EmployeeID. It is a foreign key in the Address table linking to the primary key EmployeeID in the Employee table. Thus, one record of the Employee table can point to multiple records in the Address table. This is a One-to-Many relationship.
Employee Address
EmployeeID ForeignKey(EmployeeID)
Name,... Detail,...
Many-to-many relation
Many-to-Many relationship lets us relate each row in one table to many rows in another table and vice versa.
As an example, an employee in the Employee table can have many skills from the EmployeeSkill table and also, one skill can be associated with one or more employees.
Employee EmployeeSkill Skill
EmployeeID ForeignKey(EmployeeID) SkillID
Name,... ForeignKey(SkillID) Description,...
The many-to-many relation between Employee and Skill using the junction table EmploySkill.
Individually, the Employee and EmployeeSkill have a one-to-many relation, and the Skill and EmployeeSkill tables have one-to-many relation. But, they form many-to-many relation by using a junction table EmployeeSkill.
See https://www.tutorialsteacher.com/sqlserver/tables-relations