When Captain Olimar sees a creature or feature, he jots down some notes. His, unlike Louie's notes, are very informative and provide a lot of detail. Olimar speculates quite a lot, though that is natural due to the sheer strangeness of the life forms on the planet. He also appears to be quite the biologist, if you assume that all his notes are his honest opinions.
Example
"The largest member of the dweevil family, this fearsome predator carries protective components that often exhibit offensive capabilities, an evolution that may be attributed to mere chance. Another evolutionary theory is that the chemical contents of the containers carried by the Titan Dweevil contribute to possible gene splicing. While other dweevils do not seem to choose what objects they carry, the Titan Dweevil appears to prefer shiny objects above all others."
List
In Pikipedia, the notes are written in the "/olimar" subpage of each Piklopedia creature. These are written exactly as they appear in the game, although mistakes can happen. If you spot what seems to be a typo, discuss it on the talk page, so that users who own the game might confirm if it's a typo from the subpage's editor, a typo in the game, or not a typo at all.
For a list of notes, read the category.