Nomenclature of amoxicillin
To preface this, I'm not sure if this is indeed the correct IUPAC name as per the latest 2013 recommendations. It might well be, but I don't know. However, that's not crucial to your question, which is more about understanding the name that is given on Wikipedia. Here's the structure of amoxicillin:



The amide C=O is in "acetyl", which refers to the $\ce{CH3C(=O)}$ group (see e.g. acetyl chloride, $\ce{CH3COCl}$):



The '24' is probably a typo and should instead be '2'.



Following this numbering scheme you can see that the lactam C=O is in '-7-oxo'.