It's an open access online repository for almost all the available research reviews, articles and scientific paper. Pubmed is basically showing you the abstract of papers from where you can choose whether to read a particular paper or not, and whether it would be of any value to you. And Pubmed Central is an advanced online repository which tells you about the entire full length research article or paper 

Pubmed : https://www.ncbi.nlm.nih.gov/pubmed/

Pubmed Central: https://www.ncbi.nlm.nih.gov/pmc/