Telling the difference between Psychics and Mediums is rather simple, and although some time people use these terms interchangeably it's important to note their distinctions. First and foremost, all Mediums are Psychic, but not all Psychics are Mediums. While I truly believe with enough practice and attunement everyone has some level of mediumship ability, this is generally what you'll hear said in the Spiritual community. This is because in order for a medium to function in their "role" they have to be at a certain level of sensitivity to subtle energy, often which that surpasses the level required for psychic intuition. While it's important to note that there are several different kinds, types, or styles of Mediumship, (the physical act of doing energy work as a medium), we are just going to focus on the baseline commonalties here. Your broad working definition if you will, for both Psychic and Medium.
Psychic - Is a person who is attuned and can sense, interpret, or pick up information from subtle energy within the physical plane. Often by using their intuition and ESP through the uses of the 7 clair abilities. Often receiving information on the subtle energy body/light-body/charkas/auric field, past, present, future cycles/patterns of behavior, and emotional well-beings/states of a person, place, or thing often to offer guidance or assistance in another's life, focus, or purpose and at times even to assist in the location of lost items.
Medium - Is a person who is attuned to and can communicate with the Spirit World, deceased people/loved ones, and/or animals. They act as a bridge gapper for communication and convene or conversate through energy with the Spirit Realm(s), often beyond the veil of the physical. Often receiving/channeling direct messages from Spirits to impart or relay to themselves or others.
It's important to remember that both psychic and mediums are highly sensitive to subtle energy that is considered outside of the normal realm of the five human senses, and both often utilize the 7 clair abilities. However, their area of focus is often the difference. Mediums focus on Spirit communication to deliver messages, while this can be very healing and helpful to the person still left in the physical grieving a loss, it's focus is not on the physical person grieving, but the conversation with the Spirit from beyond the veil.