If you tag someone in a post or a photo, then their friends will be able to like or comment on your post or photo, even if they are not your friend too.
The first couple of times this happened to me, it startled me.
You can click on Home, Privacy Settings, and customize your tag settings. I recommend setting the profile review and tag review to on. For the general public, I recommend setting the bottom 2 settings on that screen to off: tag suggestions and friends can check you into places.
If you are a public figure and are comfortable with others checking you into places, some choose to change that final setting on friends can check you into places to on.
Facebook has created a guide to Facebook tagging which has useful information.