Within your account you will see the option to create four different types of conversations.
Direct Message
A direct message conversation would be directly between two users. This conversation type would be a private conversation between just the two participants. This will not allow you to add new participants to the conversation. Any user with access to the messaging suite is able to create a direct message conversation.
Group Direct Message
A group direct message conversation would be between a group of users who are in a private conversation. The only difference between a group direct message is that there is more than two people inside of the group. Once again, any user with access to the messaging suite is able to create a direct message conversation
Public Channel
Public channels are conversation that can include specific users or groups of users based on their demographic data. Think of a group you may want that contains all members of Human Resources. The benefit of creating a channel compared to just a direct message is when a user is added, they are able to access all the previous message history. Keep in mind while only Messaging admins are able to create a public channel, any members in the company will be able to join the channel.
Private Channel
Private channels are conversations that can include specific users or groups of users based on their demographic data. The main difference between a public and private channel is that a private channel will not be visible to any users that are not members of the channel. This is a great option if you want to keep certain channels inaccessible from your base group of employees. This type of channel is also only able to be created by messaging admins.