In assembly modeling, you combine parts and subassemblies to form an assembly that functions as a single unit. Parts and subassemblies are connected to one another by assembly relationships. Relationships control component placement and Degrees of Freedom (DOF). You can edit individual parts or the entire assembly. You can also define a set of features in the assembly that interact with multiple parts.
When you create or open a file of the assembly type, you are in the assembly environment. Assembly tools and commands manipulate whole parts and subassemblies, which combine to form assemblies.
When you create or open a file of the part type, you are in the part modeling environment. Part tools and commands manipulate sketches and features, which combine to form parts. You insert parts into assemblies, position them, and define the freedom of movement.
You can insert existing parts into an assembly or use sketch and part modeling commands to create parts in the context of an assembly. You can create or edit parts in the assembly context, while all other components in the assembly are visible.
In an assembly, you can create a new assembly, referred to as a subassembly. A subassembly lets you group parts that function together as a single unit. For example, an air cylinder assembly is a subassembly that can be inserted into other assemblies. If you use subassemblies to build an assembly, design changes are easier to manage.
To complete a model, you can apply assembly features. Assembly features reside only in the assembly and can affect multiple parts. Assembly features often describe specific manufacturing processes such as post-machining or match drilling and pinning.