The main difference is the position of the feet and whether or not the butt is resting on the heels.
For seiza, the top of the feet must be on the ground, toes backwards, soles more or less upward. That way, the whole leg will also be resting on the ground. The butt must be resting on the upward-pointing heels. The knees should be together or at least close. Examples: post #3036564, post #3079535, post #3087563
If the soles are facing backwards, toes down on the ground, bent forwards, and thus most of the leg not on the ground, it’s kneeling. Example: post #3073158
If the butt is not touching the heels, it’s kneeling. Examples: post #3081206, post #3099960
If the knees aren’t touching the ground, it’s squatting instead.
If the toes are facing outwards and the butt is resting on the ground, it’s wariza instead.
If both legs are off to one side, it’s yokozuwari instead.
Note that these tags are often used incorrectly. There’s also many poses that ambiguously fit between tag definitions. Others are anatomically impossible, like the mix between seiza and wariza, with the butt on the heels and the toes facing straight outwards. Example: post #3085012
PS: Please wikify tags you mention with square brackets, like so: [[seiza]]
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