Difference between aldehydes and ketones
Ketones are to aldehydes as ethers are to alcohols. You could extend this to water. Is water formalcohol? Once you begin to explore the periodic table deeply enough, you will see that there is no distinctions really exist. Only trends. The same is true of organic structure. Organic chemistry is a continuum and the groups we create are constructs -- but they are not arbitrary. Especially in organic chemistry, we group things into these categories because they help us to make assumptions based on consistent trends in reactivity. Ketones and aldehydes have significantly different chemistry. There is only a minor difference between the chemistry of acetone and butanone, but there is a pretty big difference between the chemistry of acetone and acetaldehyde. 

To make a long story short, aldehydes are categorized differently because their chemistry varies significantly enough that different sorts of reagents are needed for the same types of reactions. 

Aldehydes are much more electrophilic. This is because they don't have the electron donating effect of an alkyl group. Aldehydes have much more acidic alpha-protons. Aldehydes are also more easily oxidized. Aldehydes pose problems different problems in synthesis. For Their non-acidic proton, for example can be hard to remove for substitution.

An interesting example of a useful tool for dealing with aldehyde protons is dithiane chemistry. You can use a dithiane protecting group to make the aldehyde proton acidic, substitute it, then put the oxygen back on in a deprotection step.