"der Fernseher" is the device you watch TV on, and "das Fernsehen" refers to TV in a more abstract sense. You watch "Fernsehen" on your "Fernseher".
Mein Fernseher ist kaputt = My TV set is broken
Das Internet ist heutzutage wichtiger als das Fernsehen = These days, the Internet is more important than television
There is no distinction in English. Unfortunately, sometimes a language has one word for two similar meanings, while another language has two separate words for it. If you've ever studied Spanish, it's similar to how "ser" and "estar" both translate into "to be" in English, but are used for different specific situations.