想 (xiǎng) is written with the 心 radical on the bottom and the character for "Phase, Picture": 相 (xiāng), on the top. The top character, 相, can be further deconstructed to the wood radical: 木 (mù) and the character for "Eye": 目 (mù)
Compared to 思 (sī), 想 (xiǎng) references a more whimisical, reflective and imaginative type of thinking. For example, the word for "Imagine" is 想像(xiǎng xiàng).