Basically if you rip a CD to WAV, you’re getting a 1:1 rip of the audio that takes up the same amount of space. A 75 minute CD will take up 650mb as a WAV rip.
Lossy compression means that some data is basically thrown out to improve the compression ratio. It might sound the same to most people, but something is missing, and you can’t reconstruct the original anymore. You can easily get a 10:1 compression ratio with near-CD quality audio. Ogg is a lossy format with high compression and good sound quality.
Lossless compression means that the resulting file is compressed in such a way that all of the data is still there. You can convert the compressed file back to the exact original file. The compression here is closer to 4:1 generally. FLAC is lossless and the sound is exactly the same as the source.