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.