Ashwagandha is available in several forms, and the differences come down to how they're processed, what's added to them, and how your body absorbs them.
Powder is the most natural form - it's simply the dried, ground root with nothing added. It's versatile (you can mix it into smoothies, warm milk, or food) but has a strong, earthy, bitter taste that many people find unpleasant. Quality varies enormously depending on whether it's been heat-dried or freeze-dried, and whether it's been extracted or is whole root.
Tablets are compressed powder, but to hold their shape they almost always require binders and fillers - things like magnesium stearate, microcrystalline cellulose, or talc. This means you're getting less actual ashwagandha per dose, and potentially ingesting additives your body doesn't need.
Pills is a general term people use interchangeably with both tablets and capsules, but technically pills are an older solid dosage form that is rarely used in modern supplements. Most products people call "ashwagandha pills" are either tablets or capsules.
Capsules are the cleanest delivery format. The powder is enclosed in a shell (either gelatine or, in our case, vegan cellulose) with no need for binders, fillers or compression agents. This means the full dose is pure ashwagandha root - nothing else. Capsules also mask the bitter taste, are easy to swallow, and give you a precise, consistent dose every time.
Gummies usually contain only a small amount Ashwagandha (in extract form) and mostly consists of sweeteners and binders.