Object-relational mapping (ORM) is a technique that converts data from a database or any other data storage into objects. The main goal is to separate the business logic as much as possible from the structure of the database and to reduce the complexity of your code. When using ORM, you will probably never write a query in MySQL; instead, you will use a chain of methods. Behind the scenes, ORM will write the query with each method invocation.

There are good and bad things when using ORM. On one hand, you do not have to remember all the SQL syntax all the time and only the correct methods to invoke, which can be easier if you work with an IDE that can autocomplete methods. It is also good to abstract your code from the type of storage system, because even though it is not very common, you might want to change it later. If you use ORM, you probably have to change only the type of connection, but if you were writing raw queries, you would have a lot of work to do in order to migrate your code.

The arguable downside of using ORM could be that it may be quite difficult to write complicated queries using method chains, and you will end up writing them manually. You are also at the mercy of ORM in order to speed up the performance of your queries, whereas when writing them manually, it is you who can choose better what and how to use when querying. Finally, something that OOP purists complain about quite a lot is that using ORM fills your code with a large amount of dummy objects, similar to the domain objects that you already know.

As you can see, using ORM is not always an easy decision, but just in case you choose to use it, most of the big frameworks include one. Take your time in deciding whether or not to use one in your applications; in case you do, choose wisely which one. You might end up requiring an ORM different from the one that the framework provides.