There are a number of types of Exchange Server recipients, differentiated by the RecipientType and RecipientTypeDetails properties. The following are the common mailbox-enabled recipients.
For an exhaustive list, see BOL.
https://docs.microsoft.com/en-us/exchange/recipients/recipients?view=exchserver-2019
User mailbox
A mailbox-enabled Active Directory user account.
There is an email address for the recipient and a mailbox stored in a mailbox database somewhere in the Exchange organization.
RecipientType: UserMailbox
RecipientTypeDetails: UserMailbox
Cmdlets: Get-Mailbox.
Mail contact
There is an email address for the recipient but the mailbox is elsewhere - the Exchange organization forwards email (this is not a relay, so relay permissions do not apply).
RecipientType: MailContact
RecipientTypeDetails: MailContact
Cmdlets: Get-MailContact.
Mail user
There is an email address for the recipient but the mailbox is elsewhere - the Exchange organization forwards email (this is not a relay, so relay permissions do not apply).
A mail-enabled Active Directory user account - it can be used to sign in and access resources.
RecipientType: MailUser
RecipientTypeDetails: MailUser
Cmdlets: Get-MailUser.
Room mailbox
A Mailbox-enabled Active Directory user account (disabled), representing a bookable room Rooms have a MaximumOccupancy property (not that anything seems to take it into consideration).
RecipientType: UserMailbox
RecipientTypeDetails: RoomMailbox
Cmdlets: Get-Mailbox, New-Mailbox -Room.
Equipment mailbox
A mailbox-enabled Active Directory user account (disabled), representing a bookable resource other than a room.
RecipientType: UserMailbox
RecipientTypeDetails: EquipmentMailbox
Cmdlets: Get-Mailbox, New-Mailbox -Equipment.
Distribution group
A mail-enabled Universal Distribution group (thus, not a security principal).
RecipientType: MailUniversalDistributionGroup
RecipientTypeDetails: MailUniversalDistributionGroup
Cmdlets: Get-DistributionGroup (shows GroupType of "Universal"), Get-DistributionGroupMember.
Mail-enabled security group
A mail-enabled Universal Security group (is a security principal so can be given rights to resources).
RecipientType: MailUniversalSecurityGroup
RecipientTypeDetails: MailUniversalSecurityGroup
Cmdlets: Get-DistributionGroup (shows GroupType of "Universal, SecurityEnabled"), Get-DistributionGroupMember.
Room list
A mail-enabled Distribution group containing only room resources. Used by Outlook 2010 and 2013 and by Outlook Web App 2013. Can not be created or seen using EAC.
RecipientType: MailUniversalDistributionGroup
RecipientTypeDetails: RoomList
Cmdlets: Get-DistributionGroup (shows GroupType of "Universal"), Get-DistributionGroupMember.
Dynamic distribution group
Based either on a custom query or on "precanned" search options. EAC can create dynamic distribution groups only using the "precanned" filters. EMS can create them using the "precanned" filters or a custom OPATH filter.
RecipientType: DynamicDistributionGroup
RecipientTypeDetails: DynamicDistributionGroup
Cmdlets: Get-DynamicDistributionGroup.