Examples: It's a kind of hat. = It is similar to a hat / a type of hat / looks like a hat in my opinion a kind of - a vague description, used to help explain something in general terms because you don't know the actual words. What's a helmet? Well, it's a kind of hat. You use it to protect your head. It's kind of difficult = It is a little difficult, maybe be kind of - hedging; when you don't want to sound 100% because you think the other person will judge you. Often used by young people (very self-conscious people :-) ), but also in general. It's late, and I kind of want to go home. Do you mind if I leave? I'm kind of tired. I rather like chocolate = I really like chocolate rather - used by some speakers to mean "really", "very much" before a verb or adjective. The Taj Mahal is rather beautiful. Rather you than me = I am glad it is not me rather - comparison to mean "better", mostly used with "would", prefer I'd rather go to San Diego Zoo than Disneyland. Can't think of any more examples right now, but I'm sure there are more :-) A lesson for today, this should help you :p
'Kind of' is informal while 'rather' is formal. It should always be 'rather' in this kind of sentence unless the informality of the sentence is specific and clear, which you'll notice if it is.
@Alexis.Sage That's not quite right. "Kind of " can be used formally as long as it is used correctly. "Kind of" refers to a type or variety of something and is properly used as such. "Crema Nova is a kind of marble that is commonly used in making bathroom fixtures." "What kind of meat is this?" "She is the kind of employee I am looking for." "I'm not that kind of man." To use "kind of" any other way is incorrect. "That picture is kind of blurry." "I'm kind of bored." "That haunted house kind of scared me." "It's kind of hot." The above examples are improper uses of the term "kind of," but have come to be accepted in everyday language to mean rather or somewhat. This is why it is informal to use it in that manner, but it is okay to use it formally if you use it correctly. The bottom line is that if you are speaking or writing formally, when you want to use "kind of," try "rather" or "somewhat" first. If the sentence still makes sense, "kind of" would have been the wrong choice of words. On a personal note: I do not like the term "kind of" no matter how it is used. IF you are using it to describe a degree of something; "The Tyrannosaurus Rex was kind of big," I'm going to mark you down on grammar (unless I'm subbing for a third grade class). You better be using "rather" in that case. If you are using it to denote a type; "The Tyrannosaurus Rex was a kind of bi-pedal dinosaur," I'm going to mark you down on style. In this case use "type of" or "a variety of." But that's just me.