Simple search is the main search bar you'll see when you first visit PubMed. It's intended to be used like a google search - simply enter your terms as they are. No need to add AND or OR operators for this kind of simple search.
This can be a helpful place to start. You can use the results to inform a more advanced search strategy, if needed.
PubMed does not search the full text. PubMed is simply matching your search against the bibliographic information for each record (eg title, author, abstract, journal name).
For better results, PubMed matches your search against its own subject list (MeSH, or Medical Subject Headings), and also searches your terms across common variations (eg. child to children). This is referred to as Automatic Term Mapping (ATM). Most of the times this works as you'd expect, but sometimes there are unexpected results:
It can be useful to see how your simple search was translated. To do so, click "Advanced" under the search bar:
Under "History and Search Details," click the arrow under "Details" to see what *actually* got searched.
In this example we can see that our search terms are more or less translated appropriately. It includes "huntington disease" and "genes" as MeSH terms. It also searches huntington disease and genes across all other fields.
While there's plenty of room for improvement, this might be fine for your purposes - if it is, great!
If you want more control, you'll want to move on to an advanced search.
| TIP: Using truncation (*) or quotations around search terms in basic search mode will turn off automatic term mapping. |
When you are viewing an article record, you can view a list of records for similar articles. 
PubMed uses an algorithm to compare words from the Title and Abstract fields of each citation, as well as the assigned MeSH headings. The best matches for each citation are pre-calculated and stored as a set.
You may see a few citations without the Similar Articles link, which simply means that these citations have not yet gone through the algorithm. This process may take several days.
You can navigate to this section by clicking Similar articles under the "Page Navigation" section in the right column.