Olelet is described in mishnah peah 7:4:
אֵיזוֹהִי עוֹלֶלֶת. כָּל שֶׁאֵין לָהּ לֹא כָתֵף וְלֹא נָטֵף. אִם יֶשׁ לָהּ כָּתֵף אוֹ נָטֵף, שֶׁל בַּעַל הַבַּיִת, אִם סָפֵק, לָעֲנִיִּים. עוֹלֶלֶת שֶׁבָּאַרְכֻּבָּה, אִם נִקְרֶצֶת עִם הָאֶשְׁכּוֹל, הֲרֵי הִיא שֶׁל בַּעַל הַבַּיִת, וְאִם לָאו, הֲרֵי הִיא שֶׁל עֲנִיִּים. גַּרְגֵּר יְחִידִי, רַבִּי יְהוּדָה אוֹמֵר, אֶשְׁכּוֹל. וַחֲכָמִים אוֹמְרִים, עוֹלֶלֶת:
Which is Olelet? Any [cluster] that has neither a shoulder nor a dangling portion [but rather is entirely attached to the main stem]. If there is a shoulder or a dangling portion, it belongs to the property owner; if there is a doubt, it belongs to the poor. An Olelet that is attached to the knee [the joint between branches or the stem and the trunk] - if it is plucked with the grape cluster, indeed it belongs to the property owner; if not, indeed it belongs to the poor. A single grape - Rabbi Yehudah says: It is a cluster; the Sages say: It is an Olelet.
Unfortunately, I don't really know how grapes grow, and I'm having a hard time understanding what's being referred to, even after reading this and the Kehati English. Are there pictures or a diagram of the various features of grape growth that correspond to olelet?