I am elaborating here on an issue that has come up in both my classes this semester, and was exacerbated (eh, that may be too harsh a word, but I like it) by a question I was asked today, namely, "what's the difference between Yiddish and Hebrew?"
So this is the blog-level answer, i.e., don't cite this in any sort of formal writing assignment (even if I assign it).
Yiddish is a language very similar to German. In fact, some might argue it is a dialect of German. Others might argue it's actually closer to Dutch, but then an argue can be made that Dutch is, too, a dialect of German, or that Dutch and Deutsch (=German) are both dialects of something... West Germanic perhaps. At any rate, they are both Germanic languages (or dialects; linguists don't really care about that distinction all that much). Other examples of Germanic languages are Swedish, English, Icelandic.
Hebrew, OTOH, is a Semitic language. Some other Semitic languages spoken today are Arabic, Amharic, Tigrinya and neo-Aramaic.
One of the sources of confusion is that Hebrew and Yiddish are both conventionally written using what we typically refer to as the Hebrew alphabet. Actually, if you look at the history of writing, the so-called Hebrew alphabet (e.g., אבגדה) was really used for Aramaic before it was used for Hebrew. But the Christian dialects of Aramaic (aka Syriac) were written in three different scripts (different from one another and from the Hebrew/Aramaic script).
But here's the shocking part: Languages don't have letters. Yes, yes, call me a radical (thanks for that btw). But for real, language is first and foremost an oral/spoken entity. Writing is secondary to speaking. We can fill an entire semester talking about that, so for now just trust me (or better, do some research).
So the fact that two languages use the same alphabet says very little, if anything, about the languages being related or similar to one another. Consider English and Swahili. Both use the Roman alphabet, but have VERY little in common.
The Hebrew/Yiddish connection is slightly less arbitrary than that. Both languages are/were spoken primarily by Jews, and the Yiddish vocabulary is sprinkled with words of Hebrew origin that have been borrowed due to the cultural and religious overlap. Similar cases are those of Ladino (or more precisely, Judeo-Spanish), which is written in Hebrew characters and has some Hebrew loanwords, and even some varieties of Judeo-Arabic, i.e., Arabic spoken and documented by Jews from the Middle Ages onward, written with the same Hebrew alphabet.
Similarly, the Arabic alphabet is used nowadays to write two major non-Semitic languages: Persian (aka Farsi) and Urdu. Both are Indo-European languages, and are really more closely related to English and German than to Arabic. But the fact that they're written in Arabic characters, and the presence of many Arabic loanwords in them create the illusion that they are close relatives of Arabic. But listen to Arabic and Hebrew vs. Farsi and Yiddish, or better, analyze their grammars, and you'll discover that the former two are related to one another, and the latter two are related to one another. So that the languages are classified by criteria other than the alphabets people have decided to adopt for graphically representing them centuries (if not millennia) after they began speaking them.
Okay, I can go on and on, but y'all are already yawnin'.
salaam - shalom - peace