Did you look at the documentation: http://scikit-learn.org/dev/modules/pipeline.html
I feel it is pretty clear.
Update: in 0.21 you can use just square brackets:
pipeline['pca']
or indices
pipeline[1]
There are two ways to get to the steps in a pipeline, either using indices or using the string names you gave:
pipeline.named_steps['pca']
pipeline.steps[1][1]
This will give you the PCA object, on which you can get components.
With named_steps you can also use attribute access with a . which allows autocompletion:
pipeline.names_steps.pca.<tab here gives autocomplete>