The two major kinds of theses are justificatory (or persuasive) and explanatory (or descriptive). The thesis is a sentence found somewhere in the first paragraph (often at the end) that outlines the main proposal the paper is going to illustrate.
In a justificatory paper, the author is making a claim they intend to justify. The author is attempting to convince the reader, through rhetorical devices such as ethos, logos and pathos, that one position is correct. Thus, a justificatory thesis states the argument and perhaps hints at the overarching logic the author will use to justify the claim.
Justificatory example: "When considering societal impact, World War Two was a more significant 20th century human catastrophe than World War One"
-The author will need to define "important" in the paper, as it relates to societal impact. This paper would probably use three areas (let's say: loss of human life; destruction of national cultures; engineering innovation) to compare the impacts of WW2 and WW1, with the ultimate conclusion being that WW2 had a more significant impact across these areas.
In an explanatory paper, the author is not attempting to convince the reader, but is merely discussing a topic. A good test is whether "because" can be used after the thesis: "WW2 was more significant than WW1 because more people lost their lives." As opposed to an explanatory thesis: "This paper will consider the impact of the atomic bombs on Japanese culture and national storytelling." There is nothing to defend or justify - just a blunt statement of the topic the author is going to explore. [Most explanatory theses use a similar format - "this paper, this essay, the author, etc." and use words like "explore, outline, investigate, describe, analyze"]