It's important to understand the difference between Medline and PubMed. Medline is part of PubMed; when you search PubMed you are searching all of Medline, but if you only search Medline you will miss some material that is only in PubMed.
For a truly thorough literature search you must search PubMed, not just Medline. PubMed includes very new in-process articles that haven't yet been fully indexed for Medline, and other publications that do not meet the inclusion criteria for Medline. Be aware that by building a search in the MeSH database you are by default only searching Medline, because only articles indexed for Medline have MeSH terms assigned to them.
For more details on the differences between PubMed and Medline, see here.
Citation tracing is the process of working from the list of references from a published article to retrieve the full text of articles you're interested in reading.