Shell feature is used to remove material from an existing feature and create a thin walled structure. This can be useful in creating hollow structures like pipes etc..
To start with lets create a extrude of 30 mm in length using a circular profile of 15mm diameter as shown in figure.
Now select the shell feature in the features tab pane
Now select the cylinder as the feature to create the shell out of it and select the two circular faces as shown to remove them.
Enter the thickness of the structure as 2mm and click ok. You will see the preview like
Click ok to finish the shell structure…
You can also specify multiple thickness at different faces. You can use extruded cut to achieve the same result as that of shell in this case. But when the feature is curved / bent shell is the only way to achieve the thin walled structure.