Phosphorous acid vs phosphonic acid?
The difference is that phosphonic acid ($\ce{HPO(OH)2}$) is an existing, shelf-stable compound while phosphorous acid ($\ce{P(OH)3}$) is extremely short-lived if at all and immediately tautomerises to phosphonic acid.

All the -ous acids are theoretically produced in the same manner: take the -ic acid and remove one unprotonated oxygen, leaving an additional electron pair with the central atom which reduces its oxidation state by two. So while sulphuric acid is $\ce{SO2(OH)2}$, sulphurous acid is $\ce{SO(OH)2}$.

Likewise, you can always theoretically generate the -onic acids by tautomerisation from the ‘parent’ -ous acid; one proton that previously was attached to an oxygen bonds to the central atom‘s newly-formed (when coming from the -ic acid) lone pair. Thus, sulphonic acid is $\ce{HSO2(OH)}$.

In case of sulphur, while both acids have been observed and crystal structures of both have been determined to the best of my knowledge,[citation needed] the two can still tautomerise. In the case of phosphorus, only the phosphonic acid is shelf-stable and all isolated salts have been determined to be phosphonate salts.[citation needed]

In both cases, however, organic esters have been generated in both ways. Thus, you can have e.g. triethyl phosphite $\ce{P(OEt)3}$ or diethyl ethylphosphonate $\ce{EtPO(OEt)2}$.