What's the difference between כשב and כבש? Why does the Torah use both?
I once heard a dyslexic fellow struggle mightily to read a Gemara about this. It was heartbreaking to watch.
A כבש means a "lamb", namely a young sheep.
A כשב means any member of the sheep species. Thus, a female sheep (a ewe) is a "kisba."
For instance, the Torah talks about:
"When an ox, or sheep, or goat is born ..."; it uses the name of each animal referencing the species, rather than the young.
Similarly, (Deut. 14:4), שה כשבים ושה עזים, "a young member of the sheep species, or a young member of the goat species" (the term שה means "young sheep or young goat"). In this context, the Torah is saying that the entire sheep species (irrespective of age or gender) is kosher. Contrast with Exodus 12:5, מן-הכבשים ומן-העזים, "if you're looking to bring a Passover sacrifice, the animal must be less than a year old, so get a lamb or kid goat."
According to Malbim (Ayeles HaShachar 279), the Torah uses the word כבש when talking about כבשים alone; and uses כשב when talking about כבשים as well as other species. Wherever this is not true, it means to include its tail fat with it.
Many many sources in Rishonim casually reference this as a common grammatical phenomenon, and leave it at that. For example, Radak writes:
כי מלים שוים באותיות הפוכות רבי' במקרא כמו כבש וכשב
Similarly, the Netsiv writes (Haamek Davar Leviticus 1:10) that according to Ramban, the two are absolutely identical in meaning:
מן הכשבים. דעת רמב״ן ז״ל כ״פ דכשב כבש הוא ד״א ממש
Nevertheless, Netsiv himself suggests that kesev connotes a large sheep, which is generally more than a year old, while keves refers to a lamb withing its first year, which is generally small.
אלא נראה דכשב משמעו גדול בקומה וסתמו הוא יותר מבן שנה. וכבש הוא ב״ש סתמו הוא קטן בקומה
Similarly, in his glosses to Nazir (35a) R. Sh'muel Strashun infers from the Tosafot (s.v. Rava) that they were of the opinion that the two words are identical, and he notes that this was similarly the position of Rambam (cf. his introduction to Kodashim).
Nevertheless, R. Strashun himself asserts that a keves refers to a lamb in its first year, but kesev refers to all sheep regardless of age:
אולם אני הוכחתי מק"א (ועמש"כ בפסחים סט ב) דדוקא כבש הוא בן שנה אבל כשב הוא כולל המין כולו בן שנה ובן שתים ולמעלה:
R' Shneur Zalman of Liadi explains (Likkutei Torah Emor 37b) that (Kabbalisticly) the Beis represents "wisdom" (בחכמה יבנה בית) while the Shin represents the three types of emotions (a ש has three "heads" which represent the three "styles" of emotions - kindness, severity and beauty).
So when the Torah speaks about Keves (כבש), it refers to the way sheep are "down here" - the first Sefira is Chochma (the Beis), and only from (and "after") it come the rest of the sefiros (which can be divided into the three categories).
When the Torah speaks about כשב, it refers to the spiritual "source" of the sheep, when the Shin refers to levels much above "chochma", so one has the Shin before the Beis.