Unlike viruses and worms, Trojans are not self-replicating − they rely on their apparent usefulness to spread between computers. Some Trojans work in isolation. Some, however, rely on networks, either to transmit stolen information – such as passwords, bank account details or credit card numbers – or to act as back doors to compromised computers. They allow attackers to bypass the operating system's security features and gain access to data or even control the machine over a network. Trojans have become a serious problem with Android apps.
Trojan malware programs are the weapon of choice for hackers.
Trojans get their name from the Trojan Horse that caused the fall of Troy. If you remember the story - the Greeks built the horse and left it at the gates or Troy after a long unfruitful seige. The Trojans opened their gates and brought the horse inside (thinking it a gift) - totally unaware that it contained soldiers who sneakily came out of the horse and opened the gate for the Greek army - causing the downfalll of the fortified city.
A Trojan has to be set up on the victim's computer and executed by its victim to do its work - rather like bringing the horse in through your gates. Social Engineering is used to trick you into letting the malware into your system.
Trojans often masquerade as desirable, legitimate programs (such as a screensaver), or 'free' or 'bargain' copies of expensive software. It appears fine but a trojan contains malicious instructions, therefore behind the scenes the trojan is causing damage – perhaps allowing someone else to gain control of the computer, copying personal information, deleting information, monitoring keystrokes, or using email software to pass itself on to other computers.
They can arrive via an email or get pushed on users when they visit infected websites. The most popular Trojan type is the fake antivirus program, which pops up and claims you're infected, then instructs you to run a program to clean your PC. Users swallow the bait and the Trojan takes root.
They've been around since the beginning of computing - even longer than computer viruses - but currently have taken hold more than any other type of malware.
Unlike viruses and worms, Trojans are not self-replicating − they rely on their apparent usefulness to spread between computers. Some Trojans work in isolation. Some, however, rely on networks, either to transmit stolen information – such as passwords, bank account details or credit card numbers – or to act as back doors to compromised computers. They allow attackers to bypass the operating system's security features and gain access to data or even control the machine over a network. Trojans have become a serious problem with Android apps.
RATs
Remote Access Trojans (RATs) in particular have become popular among cybercriminals. They allow the attacker to take remote control of the victim's computer. Often they are placed to move laterally (transfer to other computers linked to the one that is initially infected) and therefore they can infect an entire network.
A RAT is designed to avoid detection and is activated by the user, so a patch, firewall, and other traditional defense cannot stop it.
Cyber criminals routinely produce Trojan-building kits for sale and malware writers pump out Trojans by the millions every month. Hundreds of 'off-the-rack' RATs are available for criminals to buy in underground marketplaces on the Dark Web.
Trojans are hard to defend against for two reasons:
They're easy to write and they are
spread by tricking end-users
Anti-malware vendors try their best to fight Trojans, but there are too many to defend against.
Ransomware is an example of a despicable piece of trojan malware.
Tojans 
Trojan malware programs are the weapon of choice for hackers. 
RATs
They're easy to write and they are
