What is differences between CAT (Computer-Assisted Trasnlation) and TM (Translation Memory)

First things first we will discuss abaout CAT

- What is CAT
   Computer-assisted translation,computer-aided translation or CAT is a form of language translation in which a human translator uses computer hardware to support and facilitate the translation process.

Overview

The automatic machine translation systems available today are not able to produce high-quality translations unaided: their output must be edited by a human to correct errors and improve the quality of translation. Computer-assisted translation (CAT) incorporates that manual editing stage into the software, making translation an interactive process between human and computer.
Some advanced computer-assisted translation solutions include controlled machine translation (MT). Higher priced MT modules generally provide a more complex set of tools available to the translator, which may include terminology management features and various other linguistic tools and utilities. Carefully customized user dictionaries based on correct terminology significantly improve the accuracy of MT, and as a result, aim at increasing the efficiency of the entire translation process.

Range of tools

Computer-assisted translation is a broad and imprecise term covering a range of tools, from the fairly simple to the complicated. These can include:

• Translation memory tools (TM tools), consisting of a database of text segments in a source language and their translations in one or more target languages.
• Spell checkers, either built into word processing software, or available as add-on programs
• Grammar checkers, either built into word processing software, or available as add-on programs.
• Terminology managers, which allow translators to manage their own terminology bank in an electronic form. This can range from a simple table created in the translator's word processing software or spreadsheet, a database created in a program such as FileMaker Pro or, for more robust (and more expensive) solutions, specialized software packages such as SDL MultiTerm, LogiTerm, Termex, TermWeb, etc.
• Electronic dictionaries, either unilingual or bilingual, also known as dictorobotary
• Terminology databases, either on the host computer or accessible through the Internet, such as TERMIUM Plus or Grand dictionnaire terminologique from the Office québécois de la langue française
• Full-text search tools (or indexers), which allow the user to query already translated texts or reference documents of various kinds. Some such indexers are ISYS Search Software, dtSearch Desktop and Naturel
• Concordancers, which are programs that retrieve instances of a word or an expression and their respective context in a monolingual, bilingual or multilingual corpus, such as a bitext or a translation memory
• Bitext aligners: tools that align a source text and its translation which can then be analyzed using a full-text search tool or a concordancer
• Project management software that allows linguists to structure complex translation projects in a form of chain of tasks (often called "workflow"), assign the various tasks to different people, and track the progress of each of these tasks

And What is MT (Machine Translation) ?

Machine translation (MT) refers to fully automated software that can translate source content into target languages. Humans may use MT to help them render text and speech into another language, or the MT software may operate without human intervention.
MT tools are often used to translate vast amounts of information involving millions of words that could not possibly be translated the traditional way. The quality of MT output can vary considerably; MT systems require “training” in the desired domain and language pair to increase quality.
Translation companies use MT to augment productivity of their translators, cut costs, and provide post-editing services to clients. MT use by language service providers is growing quickly. In 2016, SDL—one of the largest translation companies in the world—announced it translates 20 times more content with MT than with human teams.

MT Systems

Generic MT usually refers to platforms such as Google Translate, Bing, Yandex, and Naver. These platforms provide MT for ad hoc translations to millions of people. Companies can buy generic MT for batch pre-translation and connect to their own systems via API.

Customizable MT refers to MT software that has a basic component and can be trained to improve terminology accuracy in a chosen domain (medical, legal, IP, or a company’s own preferred terminology). For example, WIPO’s specialist MT engine translates patents more accurately than generalist MT engines, and eBay’s solution can understand and render into other languages hundreds of abbreviations used in electronic commerce.

Adaptive MT offers suggestions to translators as they type in their CAT-tool, and learns from their input continuously in real time. Introduced by Lilt in 2016 and by SDL in 2017, adaptive MT is believed to improve translator productivity significantly and can challenge translation memory technology in the future.
There are over 100 providers of MT technologies. Some of them are strictly MT developers, others are translation firms and IT giants.

So now... Lets make the summary and start to compare them shall we?!

In terms of -	CAT Tools	Machine Translation
TIME	CAT Tools create a translation memory. This helps the translators work faster.	Computers can process machine translation instantly.
ACCURACY	Translators, who use CAT Tools as a help in translation, have high accuracy. As the translation memory is fed by a human and not by a machine.	No human is involved in this translation process. The major disadvantage is that it majorly lacks accuracy.
COST	CAT Tools are not available for free. Writers and translators have to purchase these CAT tools.	Machine Translation is free of cost. Also, machine translation is good for a couple of words or sentences. It's cheap and fast.
RELIABILITY	As humans and software, both are involved, the CAT Tools' translations have the highest reliability.	Machine Translation is not accurate and not at all reliable. Machines do NOT understand nuances, idioms, and phrases.
QUALITY	The quality of CAT tools depends on the quality of the translator's work. However, human translation is better in quality as compared to machine translations.	Machine translation technology has improved greatly. If the quantity of text is less, the quality will be good.
HUMAN INVOLVEMENT	CAT Tools have major human involvement. The translated database is stored by the translators. Hence, it is just a tool for quicker and better translation.	There is absolutely no human involvement. For translation and localization, human translations work best.

in the comparison above we can drag a conclusion that CAT and MT has advantages and disadvantages of each. 

Source : http://www.cmmlanguages.com/cat-tools-vs-machine-translation.html - https://en.wikipedia.org/wiki/Machine_translation - https://en.wikipedia.org/wiki/Computer-assisted_translation