The word machinery is used when referring to a group of machines, as opposed to a single machine. "The factory caught on fire last night and all of the machinery has been damaged." "There was a small fire yesterday, but only one machine was damaged."

Melissa's answer is right, but it may leave room for another confusion because machinery is not the plural of machine. You can see that in her answer, because she also talks about a "group of machines." Machines is the plural. Machinery is a collective noun, and can function more as a category description, because often it includes things which might not individually be called a machine. So when someone says "there is a lot of machinery in that room", they could be referring to both machines and other equipment that is related to the machines. As a category, it can also be very general. So "textile machinery" might not refer to any particular machines, but might be a general reference to the kinds of machines that might be used in the textile industry.

Great answer. At the sight of this question, I had an instant feeling that the asker is probably Chinese-speaking: because this is a problem a lot of Chinese people, I for one, will be faced in some stage of their English learning. Collective nouns are a common catagory of English languae so that native English speakers have a natural sense of its concept and usage. Chinese lanugage has it too, such as, 机械machinery，船只shipping, 纸张 paper, 牲畜 livestock, 诗歌 poetry, 家具 furniture，but it is so familar to us that we do not even have this concept in mind, if the grammars do not tell you about that( and it's the normal situation in our basic grammar teaching), hence we offen do not easily associate the the category of English to that in our lanuguage. English have 3 ways to convey the concept of group, as far as lexicology is concerned：1) collective noun (CN), machinery 2) uncountable noun, furniture 3) the pluaral: apples. In Chinese we have only the first way, as exampled above. Athough the meaning of CN in English and Chinese are similar, their usage is far different. In English, machinery ≠ machines, shipping ≠ ships. You cannot say several information(s)/furniture（s）/slang(s), and you have to say several PIECEs of furniture/information, or several slang words(phrases), while in Chinese you can say 几头牲畜, a few head of livestock, 几条信息，几件家具， 几条俚语，but you cannot say 几片纸张，几条船只. If one wants to explore the specific usage of collective nouns and why there are such differences, they need to turn to the semantics: grammar and pramatics cannot be separated from semantics.