I need to use liquid to create templates but might only be able to access the Twig template language. Are these two languages basically the same thing, or are there differences in the language that would render it difficult to use Twig as a replacement for liquid?
The basic concept is same for both Twig and Liquid, but some of tags and filters are different, so if you're asking if it's take'n replace, then the answer is no.
Note below the line - there is good documentation for both available online and I think you should definitelly do your own research before asking such a question. You can see the differences in couple of minutes...