I'm curious about how are cameras are categorised. For example, d90 is said to be an intermediate level DSLR. D3000, 3100, 5000, 5100 are known as entry levels. What features do they need to possess to be categorised under a certain category. Please do not flame me if this is a redundant question. Thanks to all!
It is very rudimentary.
For example, for some funny reason, I have seen Pentax K-5 being placed together with Canon 550D before, because of the "megapixel" count. This is despite the fact that the Canon 550D is more or less a mid-entry level camera, while the K-5 is far more advanced. That same article naturally went on to say that the 550D was cheaper, which was ridiculous.
It really depends on how credible the site/magazine/writer is.
These days various companies have very segregated levels. It used to be simpler - with entry level, mid-range/prosumer and professional, where professional would be the flagship cameras (usually, based on my understanding). These days, just take a look at the Canon lineup for example, you have the Canon 1100D, Canon 600D, Canon 60D, Canon 7D, Canon 5D M2, Canon 1D Mark III. The first two differentiate the entry level market further, the Canon 60D/7D differentiate the mid-range/prosumer market further, and well, I'm not sure where the Canon 5D M2 falls, while 1DM3 is the flagship.
Same for Nikon, naturally. It's just really really messy, and I can't even keep track of Sony with their translucent mirror offerings and the normal DSLRs (and they have a huge slew of normal DSLRs, it seems).