Like Janszoon said, they're very similar and especially as you get closer to the origins of death metal. A lot of the same qualities can be found in almost all early forms of extreme metal. What sets them apart, I think, is thrash's foundations being rooted in NWOBHM style metal, or power metal, which is typically more melodic. Thrash was a heavier variety of "speed metal" (a more sped up version of power metal), usually with more controversial/violent lyrical content and with faster/heavier (more distorted) musicianship.
Death metal basically takes the heaviest thrash and does the same thing with it that thrash metal did with power metal. Lyrics are typically very graphic, musicianship is even faster and heavier, etc.
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