要 means to want to, to ask to or will do something. wo yao shui jiao can also means i will go to sleep. 想 means think, would like or want(+sb) to do something. in general, 要 can be put before a noun. wo yao yi bei shui(我要一杯水), but you cannot say wo xiang yi bei shui(我想一杯水). 想 before a noun or a pronoun means miss, wo xiang ni le(我想你了). 要 is to act, i want to be given something. 想 is often show a thought, idea or hope which is an act in mind. i only want to do so, but maybe cannot do. 要 is to need, 想 means to want. 要 is an instant act, 想 as thinking can be continuous. 你要干什么? - what are you going to do? 你想干什么? - what do you want to do? 你在要什么? X. 你要什么? V. - what do you need? 你在想什么? - what are you thinking? 你想要什么? - what do you want? here, notice, 想要 means to want, but we often say a single word to express our hope. as above, 想 can express to hope, 要 can express to need, and the both two can express to want. maybe, you have seen 要想....,就必须...., here 要想 is different from all above, it means so as to, in order that.