I started rock climbing 8 years ago, but haven't been actively pursuing the sport for the last few years. There are different rating systems. In the United States, climbs are rated from 5.1 to 5.14d, with 5.1 being the easiest. In fact, 5.1 to 5.4 are all pretty simple. I always climbed with a rope, and because I started rock climbing in Ohio, we often used a top-rope since we could walk up the back of a small cliff and secure a rope. If you take up rock climbing as a hobby, it would be best to take classes. It is very important to be safe. My brother and I took a class at Grand Teton National Park with Exum. Also, pick a climbing partner you trust. They, literally, have your life in their hands.
Besides just climbing a rock face, this sport also includes rappelling(you have to get down the rock somehow), belaying, knot-tying, and other stuff. Do your research or else you're just trying to kill yourself and ruin the sport for others.

Update/Clarification: 5.1 to 5.4 is pretty simple, meaning that you probably don't need a rope. However, even an easy climb can seem scary when you're out on a ledge.
5.5 to 5.9 is an intermediate level. I definitely needed a rope, and 5.9 was about as well as I could climb. I definitely could not lead a 5.9. (to lead a 5.9 you should be able to easily climb 5.9)
the double digit classifications are really hard, and I'm not quite sure how they determine it. (The letter part only goes from a-d)