What's the difference between desu and yo?

I read that 'yo' is used to assert something e.g. おいしいよ. Can you not also end it in 'desu'? Are they interchangeable? Do they have completely different meanings, or is it simply a difference in formality/politeness and strength of emphasis/how strongly you are asserting something?

Edit: I know that desu is the polite form of da.

Edit: Added the question about whether the two are interchangeable.

The two are completely different. 'desu' is a polite form of the copula while 'yo' is a sentence final particle, in the same family with 'ka' and 'ne'.

All of these are valid ways to remark on something being delicious,
おいしい
おいしいよ
おいしいです
おいしいですよ

This seems like a strange point to be confused on, the first couple chapters of any decent textbook should make the usage of です and sentence final particles pretty clear. What are you studying from?