You can make two broad types of change within Salesforce:
In this article, we'll focus on declarative changes: what they are and how you can make them.
What is a declarative change?
Salesforce has a motto of "clicks not code" when it comes to development on the platform. This approach encourages users to minimize code use and make changes through the platform UI instead.
These kinds of changes are known as declarative changes, and they form the mainstay of many users' customization of their orgs.
The benefits of declarative development
By allowing users to make changes without writing code, Salesforce allows anyone to begin customizing the platform, even if they don't have a software development background. This opens up the platform to a much broader user base and encourages many more users to get involved with development.
The approach of encouraging "citizen developers" is one of the big draws of the Salesforce platform over more traditional CRMs. It allows companies to develop new features and respond to change faster than would ordinarily be possible.
How declarative changes are made
All declarative changes are made through the UI in an org. They can be as simple as clicking a button to enable a feature, or as complex as building workflows to automate entire processes.
Here are some examples of the sort of changes you can make declaratively:
Adding a new custom field to the Account object.
Changing a page layout to control how a page displays to a user.
Editing a permission set to control which users can view a field.
Adding a new value for a value set.
Want to know more?
You can learn more about declarative changes in the excellent Salesforce Trailhead modules for admins here.