练习 is practice. 排练 is rehearsal.

练习 completely equals "practice" 排练=rehearsal, practice for a show or something,

排练means 练习for a particular show or public performance，mostly there're more than one people involved，but 练习 could mean 排练in some terms，but most of the time it means practising for some skills and only you is involved, it's also good to say:我和他一起练习，when the object you're practising is not a public performance.