A student asked me an interesting question about what is a SPFieldLink.
The correct answer is that a content type (SPContentType class) doesn’t have columns (SPField) (a list does), but the content type has just references to fields (Site columns) and this references are SPFieldLink.
SPContentType as a FieldLinks collection that returns all the SPFieldLink (s).
A quick look at the sdk confirmed this point of view, but I noticed that there was also a Fields property in the SPContent class (of type SPFieldCollection).
Reflector and the following article (see the postscript) confirmed the fact that this Fields property is just a merged view of FieldLinks.
There is a 1-1 correlation between the items of FieldLinks and those of Fields.