clear definition about gethit and pinhit

I have been working as dba several years and I've read hundreds of papers about  library cache in terms of efficacy and hit ratios .

However none of that papers offered me a clear meaning  of gethit and pinhit ratios.

Some of those articles were , in fact are, simply copies from others official documents with a none or tiny changes.

Others documents talk about the relationship between gethit and parse phase and pinhit  and execution phase but they don't define the relationship between both ratios and  whole  processing of sql.

After reading a lot of information on the subject I have my own conclusions but with many doubts. For example , what factor makes gethit or pinhit counters increase ?