Very similar Nowadays = the period of time that is happening now; any continuous stretch of time including the moment of speech. These days = the period of time that is happening now or happened in the past However, usage is different. Using DancingBear's example, "These days I HAVE BEEN too busy with work to write to you." "Nowadays I AM too busy with work to write to you."

very similar meaning nowadays = the present, the current days these days = the current days

Although the meaning is similar, they are not always interchangeable in usage. ("Nowadays" means "in these days.") For example, you could say, "These days I've been too busy with work to write to you." It would not be appropriate to use "'nowadays" in that context. At least, it would not sound natural to me.

They're interchangeable / near synonyms.