There are two methods for finding systematic reviews in PubMed.
1) From the PubMed home page, select the link for Clinical Queries in the middle of the page. Input your search terms in the search box. Once you have your list of systematic reviews you can still use 'Limits' to further restrict your results, if necessary. Or,
2) After you have entered your search terms and arrived at references on your topic of interest, you can go to 'Limits' and select "Systematic Review" from the Subsets box under 'Topics'.
Your results will include both Cochrane Reviews and other systematic reviews and meta-analyses.
A meta-analysis is a systematic review in which the actual quantitative results from the studies included are combined to enhance the synthesis of research results in a quantitative as well as qualitative manner.
You can specifically search for systematic reviews in PubMed by using the Clinical Queries link from the PubMed homepage.
Here is the definition of a systematic review from the AHRQ: It is a "summary of the clinical literature. A systematic review is a critical assessment and evaluation of all research studies that address a particular clinical issue. The researchers use an organized method of locating, assembling, and evaluating a body of literature on a particular topic using a set of specific criteria. A systematic review typically includes a description of the findings of the collection of research studies. The systematic review may also include a quantitative pooling of data, called a meta-analysis."
Systematic Reviews are at the top of the information hierarchy as they provide filtered, evaluated information and save you the time of analyzing each research study. Cochrane Reviews are an excellent example of these.
You can find systematic reviews via PubMed by using the "Clinical Queries" search function that is an option from the home page of PubMed, or you can use the Systematic Review limit to apply to particular search results.