I think at one time or another we have all asked this question. In my experiences, yes they do exist. They do not come with wings as depicted in hollywood movies. Not all angels are unseen. There are people that sometimes angels work through to get a message to another person or help another one in need. This is a more direct way in which they may serve you.
Other times they work in silence but their work is nothing short of miraculous. There are countless stories of people who should have been dead because of an accident they were involved in but somehow walked away with very little injuries. Most just say, "they were lucky". It's much more than luck. There were unseen forces at work that made this happen.
How many times did you "almost" hit that car, or a car "almost" hit you??? The "almost" didn't happen because there was a guardian angel by your side at that moment. These acts of mercy happen over and over again but yet they go unnoticed.
It can even be a simple reminder because maybe you were wallowing in self pity and misery and then someone crosses your path in worse condition or state of mind than you. This a gentle reminder of their presence and to give thanks for all of your blessings.
Pay attention to all the ways you are being guided and protected on a daily basis. It is easy for the logical mind to disregard it. Trust what you know in your heart to be true.
Guardian angels can be loved one that have crossed over, angelic beings, ancestors, friends or any spirit that is on the other side and is assigned to this role.