相信 (xiāng xìn) means 'to believe' — it emphasizes trust in the truth of a statement or in someone's character/ability. It is stronger and more emotional than 认为 (rèn wéi), which is a neutral 'to think/hold an opinion' based on reasoning. 想 (xiǎng) can also mean 'to think/believe' but is more casual and often used for personal guesses or wishes.