The Spanish verb querer literally means, "to want." When applied to objects or other verbs, its usage is pretty straight forward. Quiero una bicicleta: I want a bicycle. Quiero correr: I want to run. Quiéres sopa? Do you want soup? …and so on.
Things get a little more complicated when you apply the word "querer" to people. An innocent te quiero usually means "I love you," but follow that up with a suggestive verb–or just silence and a meaningful look–and you've implied something much more risque.
Wondering how to sort through the different interpretations of querer vs gustar? (We'll get to gustar in just a moment) Context is the key. You're most likely to encounter te quiero as an expression of affectionate love. Family members, good friends, and even companions or lovers may use this phrasing to express strong affection.
Te deseo, which literally translated means "I desire you," is a much more direct way of expressing sexual or sensual desire. If someone is using te quiero as a sly "come hither" phrase–or as a command, like Uncle Sam–the context, tone and inflection should make it obvious. Likewise, if you're looking to use this phrase, be aware that it's the context of the message and how it's delivered that will make the meaning clear.
Gustar, as with querer, has different connotations when applied to objects or to people. When applied to objects the intransitive gustar literally means "to be pleasing." So Te gusta el té? literally translates to "Does the tea please you/bring you pleasure?" As you may already know, not all phrasing translates directly from Spanish to English or vice versa, so Te gusta el té? would usually be phrased, in English, as "Do you like tea?"
Fine. Simple. Right? Not so much when you throw people into the mix. As in English there are many shades and nuances to the concept of "liking" someone in Spanish. While usage will vary from region to region or, perhaps more importantly, from personality to personality, gustar has an alternative literal translation: To please or to bring pleasure. This is most frequently used in relation to people to convey a sense of pleasure or gratification, quite possibly sexual. It's not typically applied to objects in this sense, but one could make a pun or joke by doing so. It's all in the context and delivery.
Head to Head
Let's review: The Spanish verb querer, when applied to objects, means "to want." When applied to people it usually connotates affectionate love, but can be used as part of a phrase to express physical or sexual desire.
The Spanish verb gustar literally means to be pleasing or to bring pleasure. When applied to objects, this is usually perfectly innocent. Te gusta la bicicleta? is less likely to be translated literally (Does the bike bring you pleasure?) as much as figuratively (Do you like the bike?). When applied to people, however, the literal interpretation may be correct: Me gustas, or "you please me," can be interpreted in any range of desire and physical or sexual connotations. Or it may still be tinged with innocence. Again, context is the key.
What do these Spanish verbs have in common? They both change connotations when applied to objects or people. They can both be used to express perfectly innocent concepts, or to show physical or sexual desire. It's even possible to get the two confused: Te gustaría…? translates literally as "Would it please you to…?" but is more likely to be interpreted as "Would you like to…?" Sounds eerily like Quiéres…? or "Do you want to…?" doesn't it?
If you're still having trouble choosing between querer vs gustar, just remember that the subject matters. If you're talking about an inanimate object or concept, querer becomes want and gustar becomes like. But if you're talking about people, querer becomes love, in either a familial, friendly or romantic sense (depending on the context and how you say it). Think of gustar, when applied to people, as more like saying "please," as in "he pleases me," with all the extra interpretations the English phrase invites.