Medium and media have the same meaning. The medium refers to the type of way of communication or entertaining someone or the artistic expression or publication of something. The medium of these things always refers to a group of items that fall into this category. If you are talking about one thing, then you use the word medium.
If you are using many of these things, then you use media. This is because the word medium is singular in form, and the word media is plural in form. Most of the time, people only use the word medium even if they mean the plural form. That is because both the term media and medium can be plural, but the word medium is only singular.