Do you have a slow or backed up bathroom drain? Have you tried drain cleaners or plunging with limited or short term success? Try following these steps to fix the issue and save yourself time and money.
First thing, STOP using drain cleaners of any type in any drain or plumbing. Though they may clear the drain in the short run they will damage your plumbing and cause other more costly issues down the road.
Second place a bucket or small bowl under the plumbing, if this is not possible due to not enough room place some rags or towels under the plumbing.
Next, under the sink on the back side of the drain is a screwed- on cap that holds the stopper lever in place. Unscrew this; some times you will need pliers and sometimes you can do it by hand. You will want to put something under the stopper in the sink so it doesn't close, making it hard to remove. After the cap is unscrewed, remove the cap and pin from the pipe and pull the stopper out from the sink. If hair was the issue (in most cases) it will be attached to either one of these. Clean them off and replace, making sure to put the pin back through the hole at the bottom of the stopper and to tighten the cap back up fairly tight so you don't have a leak.
If you'd like to take it one step further, while those parts are still out, remove the sink trap (the U-shaped pipe at the bottom of the plumbing), this will have water in it and should only be hand tight but may require pliers because of corrosion. Once the trap is removed, ball up some paper towel and push it through the drain from the sink down using a stick or something smaller than the drain. Be careful not to have the paper towel so thick it takes a lot of force but not so thin it doesn't clean the sides of the pipe. You may want to do this a couple of times until the paper towel comes out clean.
Once everything is clean just put it back together. You should only have to hand tighten but some times you may need to give it a little extra with the pliers.