The primary difference between Thinkorswim and TD Ameritrade is that TD Ameritrade is an online US broker. Whereas Thinkorswim is an online trading platform powered by TD Ameritrade.

Thinkorswim is an online trading platform by TD Ameritrade used to trade financial assets. This platform is designed for self-directed stock, options, and futures traders. A thinkswim suite offers a seamless trading experience with advanced and essential tools.

You cannot open a TD Ameritrade account in Canada unless you are an American or have US citizenship. So, if you are a Canadian citizen, you cannot open account with this US stock broker.