It depends. The description post by Aerocat is a good one that covers most moderate turbulence. Slighter versions also exist as do the more extreme. As somebody has mentioned drink, let's go through some ride qualities. The majority of time you fly we fly with the seat belt sign off. If you had a totally full cup of tea it would not spill. The next level is a full cup will spill a little every now and again. In our part of the world the rough stuff is where a 1/4 full cup might spill in cup holder. But I ask you this: would you have a open cup of hot tea in your car? In some parts of the world there are more extremes and generally it doesn't last forever. We don't like it, the cabin crew don't like it and neither do the passengers, so we try and get out of it. The plane doesn't really care.
Unfortunately, the some of the rough stuff is described by passengers as being like a roller coaster ride. The reality is that unlike a roller coaster, most rises and falls are just a few feet but the passengers have no way of knowing and exaggerated claims go unchallenged. You will often hear descriptions of "air pockets", the word "plummet" and "I thought I was going to die" when used to describe a what passengers believe is a rough ride. Amazingly, time after time these very same people are safely delivered to their destination just like a James Bond Martini - shaken, not stirred.
My suggestion is to always fly with your seatbelt on, mine always is, wherever I sit. When the bumps start, down your drink. Then relax and go with the flow. I also find gentle bumps have the ability to rock me to sleep.
Enjoy your flights.