分类就是根据对象的不同特点，把它们区分为不同类型的方法。分类中常用的表达如下：

...is made up of...

...range from...to...

...consist of/comprise/includes...

...fall into several...groups：（1）...（2）...（3）...（4）...（5）...

...separate into...groups...One group...Another group...

...be further broken down into（divided into）...

subclasses：...

...be subdivided...

完整的分类叙述包括三方面的内容：被分类的事物；被分成的类型；分类的依据或标准。

例1：

Insects associated with corpses may be divided into several groups according to their functions：（ⅰ）those already on the body prior to death，e.g.fleas or lice；（ⅱ）necrophagous species，the scavengers，principally flies；（ⅲ）predators which feed on the other insects attracted to the corpse，e.g.staphylinid beetles；（ⅳ）omnivores or generalist feeders that will consume some of the corpse and attack the other insects on it，e.g.wasps；（ⅴ）vagrant species or passers-by that are attracted to the warmth or moisture of the corpse but take no part in its decomposition.

但在实际写作中，分类说明往往只包括前两方面的内容，这是因为我们常常可以通过上下文判断出分类的依据或标准，故作者略去不写。如例2和例3：(www.daowen.com)

例2：

The universities will tend to separate into two groups.One group will estimate that their chance for grants to study fusion hydrogen energy systems is good.They will strongly support the concept.Another group say that the whole plan is not feasible because the necessary research has not been performed and it will take 50 years to understand the problem.

例3：

The term“fossil fuels”encompasses a spectrum of mineral organic compounds extracted from the earth.They range from solids to liquids and gases，and they include coal，petroleum，shale oil，tar sands and natural gas.

在分类中，有时我们需要把被分成的类别根据另一原则再分成更小的类别，更小的类别还可再进一步细分，我们把这种逐层分类称为多级分类。如：

例4：

Today's inks are divided into two classes:printing inks and writing inks.Printing inks are further broken down into two subclasses:ink for conventional printing in which a mechanical plate comes in contact with or transfers an image to the paper or object being printed on；and ink for digital nonimpact printing，which includes ink-jet and electro-photographic technologies.

例5：

Color printing inks are made primarily with linseed oil，soybean oil，or a heavy petroleum distillate as the solvent（called the vehicle）combined with organic pigments.The pigments are made up of salts of multiring nitrogen-containing compounds（dyes），such as yellow lake，peacock blue，phthalocyanine green，and orange.Inorganic pigments also are used in printing inks to a lesser extent.

分类叙述并非一定要通过使用分类词语来提供分类信息，我们还可以利用修辞手段（如：平行结构，对比等）含蓄地加以暗示。请看下段：

例6：

Plasmas can exist in the form of solids，liquids，or gases.Fluorescent lamps are among the many common plasma sources used every day，to convert electrical energy into UV radiation（Hg wavelength:253.7 nm）which is then absorbed and converted to other wavelengths by a phosphor powder coating on the lamps'glass wall.Laboratory and equipment plasma sources include inductively coupled plasma（ICP）sources，cold cathode sources，electron-cyclotron-resonant sources，end-Hall sources，Kanfmann ion guns，and radio frequency（RF）sources.