What do PMID and DOI stand for?

PMID stands for PubMed ID. It is a unique number applied to every article indexed in MEDLINE. You can find it in PubMed at the bottom of every citation:

Example:

In Ovid, PMID's are called "UI"s.

Example:

DOI stands for Digital Object Identifier. It is another unique number applied to articles by certain publishers. Look for it on the bottom of the first page of an article, or in a publisher's online citation.

Here is an example from the journal Hypertension: