The main difference between the active and passive is that in an active sentence, the agent (the person who does the action) is the subject of the verb. For example: Juan escribe un libro and los chicos hacen los pasteles con chocolate.
However, when we use the passive, the reciever becomes the subject instead of the doer. It can be formed mainly in two ways:
It is important to remember adjectival agreement with this construction in terms of gender and number.
It is important to remember that 'se' takes away the subject - it refers to 'them' so the subject is not needed.
In these passive sentences, the doer of the action (the agent) is no longer the subject and instead it is the recipient.