An acquaintance is often called someone you "know in passing" and means that you may see and interact with the individual on an occasional or even regular basis, but they are not your actual friend. Unless the person acknowledges you as a friend (and you spend time together outside of the function or event that brought you together), you are an acquaintance. The distinction to friendship comes when a personal relationship has developed.
You may be an acquaintance of someone for many years without a true friendship developing. Or, it can be the first step in getting to know a future friend. It depends on the two people involved and the circumstances of events that bring them together.
Mentors are people who share their knowledge and experience with us. It is not unusual for a mentor to be older, and as a result look at life a different way. Mentors help us navigate our career and personal choices, as well as introduce us to new people. A mentor can become a friend at some point, especially when both people are more on the same level. However, a mentor may sometimes never be a real friend. They are nonetheless an important part of our lives.
Online friends, or people that you know solely through the Internet in some way (forums, Facebook, Twitter, etc.) can enhance your life in many ways. Online friends can feel as if they are "always there" because you interact with them through the Internet, where Facebook pages and forums are available round the clock.
Online friends can support us and can add value in our lives, but they cannot replace one-on-one interaction with a real-life friend. However, more and more sites are helping people meet online and then continue their friendship in the real world.
Someone who you have a personal relationship with (you've spend time together and bonded) can be considered a friend. Your friendships may be slightly different from each other, and even include different levels. You may also start out as good friends with someone, but then drift back to being occasional friends at some point in your relationship. Friendships are fluid and changing, so it's not unusual to see them go through periods of time where the people involved feel very close to each other, struggle, drift apart, and get back together. No two friendships are the same. (Here's more on the definition of a true friendship.)
A good friend is someone you would consider part of your inner circle. These are people who know the most about your life, and have likely been through a few ups and downs with you. You may have several friends and one or two people you would consider "good friends." Good friends are generally those you see and talk to the most often.
Best friends can be referred to as BFFs, bromance buddies, or even "besties." These are people who you are very close to, and who you spend a great deal of time with. They are the first person you call when you're excited, hurt, or just want to chat. Best friends can often feel like family because they are there for us during the most important moments in our life. Not everyone has a best friend, which is okay. Some people may have one best friend and no others, which is also fine. It all depends on your personal preference and what makes you happy.