Pipeline is nothing but the flow of different components in Informatica. A mapping in Informatica may contain Source, Transformations , Targets connected together, all these together will be considered as a pipeline.
There will be many such pipelines in a single mapping . One pipeline can be connected to another pipeline also; in that case it will be considered as a single pipeline
To order the pipeline, there is a option in Designer i.e. Mappings>Target> Load Order.