Beware comparison of ranks by the same *title* (i) across different services; (ii) when referring to ranks as opposed to appointments; and (iii) across different nation's armed forces
The classic example of this is Captain. This is because an Army Captain and Navy Captain have very different status and are not equivalent. A Captain in the navy equates to an Army Lt Colonel for example. Moreover, an officer might have a job title which sounds like a rank but is not. For example, the Captain of a ship might not hold the rank of Captain. In this case the "appointment" is Captain but the rank may be something else, usually lower. This is the classic confusion that you would get in Star Trek, which they overcome by giving all the (ships) Captain the *rank* of Captain also!
In terms of different nations, again the ranks may not directly compare. For example a US Army Sergeant (basic level) would not hold as much status and responsibility as a British Army Sergeant. Of course there are several grades of Sergeant in the US Army that do not appear in the British rank structure at all. So beware comparison charts (I have seen them on the net) that draws comparison merly by title.