Identity and Access Management
IAM is a web service that helps to securely control access to AWS resources. In order to operate securely when handling with multiple developers, projects a good access hierarchy is compulsory.
Link to the docs
Users
Root
When creating a AWS account the first user is the root user, which owns the account, has all access rights. This account provides the billing information, has the ability to change the password of all other sub-users. This level of access can be dangerous when handling with lower-level tasks, so it's recommended to not use the root user credentials for everyday development/access.
Administrator
The Administrator user has almost the same access rights as the root. The biggest exception is that the Administrator is not the owner of the account. An Administrator can be used for managing the rest of the Accounts with a extra level of security.
Developer
Multiple developer accounts can be created. They do have access to the certain resource in which they are building the desired Application.
Groups and Policies
Summed up, those two sections are used to an easier management of different resource distribution.
Warning
This summary is only a rough overview of the complex functionality of AWS IAM. In order to get a deeper understanding reading the docs is mandatory!