a user, that is a organizer can create conferences, but can also participante in conferences
First, lets consider the User - Organizer relationship:


An Organizer is a User but a User may or may not be an Organizer


This is a one-to-one (1 - 0..1) relationship. One way to implement it is by having the PRIMARY KEY of Organizer being a FOREIGN KEY that references User:

User: 
    user_id         PK
    name
    username
    password 
    email

Organizer: 
    organizer_id    PK  FK -&gt; User 
    description


Now, for the Conference, it seems there are 2 relationships:


An Organizer can organize many Conferences; a Conference has exactly one Organizer.


This is one-to-many relationship as you correctly noted. The common way to implement it is to have organizer_id in Conference:

Conference
    conference_id   PK
    organizer_id       FK -&gt; Organizer
    name
    description



The relationship between User and Conference is probably many-to-many. A User can participate in many conferences and a conference can have many participating user.


We need a junction table:

Participation
    conference_id   PK FK1 -&gt; Conference
    user_id         PK FK2 -&gt; User