The main thing about marginalia is that they are written in the margins 
Marginalia can be text or drawings.
The term 'gloss' is used for notes in manuscripts that are translations or clarifications of words. Quite often these are interlinear (written between the lines) rather than marginal.
Pen trials are more commonly on the last pages of manuscripts, and can also be either text or drawings.
Finally, a scribe occasionally put a personal remark after the end of a text, after he finished.
When browsing through on-line manuscripts, one very frequently runs into examples of all of these.
Occasionally, entire books are written in the margins, see for example here:
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There are plenty of interesting pen trials at the You are not allowed to view links. Register or Login to view. . (Several pages of them).
There are also some interesting blogs about this. You are not allowed to view links. Register or Login to view.may be a good start.
Marginalia can be text or drawings.
The term 'gloss' is used for notes in manuscripts that are translations or clarifications of words. Quite often these are interlinear (written between the lines) rather than marginal.
Pen trials are more commonly on the last pages of manuscripts, and can also be either text or drawings.
Finally, a scribe occasionally put a personal remark after the end of a text, after he finished.
When browsing through on-line manuscripts, one very frequently runs into examples of all of these.
Occasionally, entire books are written in the margins, see for example here:
You are not allowed to view links. Register or Login to view.
There are plenty of interesting pen trials at the You are not allowed to view links. Register or Login to view. . (Several pages of them).
There are also some interesting blogs about this. You are not allowed to view links. Register or Login to view.may be a good start.