前 refers to the "front" of tangible objects (eg, a car) or the "forward" direction, but when it refer to time, 前 means "～ ago" or "previous(ly)".
- 10年前 ten years ago
- 前に聞いた話 a story I heard a while back
- 前の記事 previous article
- もっと前の出来事 an event that happened even before it
So when I see a button saying 前 on a tutorial site like TextFugu, I would expect it's the link for the previous (older) post/chapter/lesson. Apparently the 前 button in your page links to the next (newer) item, which I think is strange.
However 前 can be a bit tricky because it refers to newer items on a page where new items come first (e.g., blogs, search results, ...). On the question list of Japanese Stack Overflow (sorted by newest to oldest), the
次へ button corresponds to older questions, and
前へ corresponds to newer questions. Well, you can say
prev corresponds to 前 anyway.
後 means "rear" (spatially) or "later" (temporally). I don't see it often on navigation links, but it should be a link for the newer item (the item created later than the current one).
Here are other common and unambiguous phrases found on typical "old/new" links.
In addition, 戻る ("back") and 進む ("forward") can be used on a tutorial- or novel-type site. Perhaps TextFugu have clumsily translated "back/forward" to "後/前" instead of "戻る/進む".