Each store has their own sizing charts, so buying the right size online can be difficult. Each retailer caters to a specific target audience, so sizing schemes, and sizing names can vary from store to store. Vanity sizing is also a huge reason why you're a different size in different stores.
Every country will have their own sizing system when it comes to clothing. A size 12 in the US is a size 14 in the UK, but even then the fit could be wrong. Depending on the country and their sizing scheme, sizes between countries will vary. Measurements are what allow you to fit in clothing, not sizing names.
You can find out your body shape based on your proportions. Body proportions make you the most similar to a specific "shape". By measuring your hips, waist, bust, and shoulders, according to the ratios of your body, you will be able to identify your body shape.
Sizing charts are meant to help you pick your best fit in a store. The best way to find the best fit is to measure your bust, waist, hips, and inseam. Use store provided charts to match size names to measurements of your body parts. Finally, select the size that best overlaps with your measurements.