A Beta Reader also pre-reader or critiquer, is a non-professional reader who reads a written work, generally fiction, with the intent of looking over the material to find and improve elements such as grammar and spelling, as well as suggestions to improve the story, its characters, or its setting. Beta reading is typically done before the story is released for public consumption. Beta readers are not explicitly proofreaders or editors, but can serve in that context.
Elements highlighted by beta readers encompass things such as plot holes, problems with continuity, characterization or believability; in fiction and non-fiction, the beta might also assist the author with fact-checking.
I would always suggest talking to a Beta Reader before sending them anything. You want to make sure you and they are a good fit together. You also want to make sure they are comfortable reading what you write. No need giving people nightmares over an in depth Erotic scene or an in depth murder scene. A lot of people start off using friends. This is fine and great but be sure you are comfortable hearing and telling the truth to your friend without it affecting your friendship.
From word of mouth one Beta Reader that many people have enjoyed working with is Sarah