DHCP is a dynamic host configuration protocol server is used to assigned the ip addressed to the host computers . As you already know that every computer who wants to go on Internet must have a IP address which is 32 bit numerical number . When you connect internet your ISP assigned you a private IP address which can be 192.168.1.0 or etc but after you go on internet or access any website your private ip address automatically changed and become public ip address remember you can not access any website with a private ip address .Network administrator assigned a DHCP scope pool in DHCP Server which can be installed in ISP Window server OS or Router also can be used as a DHCP Server . How computer get the ip address when you connect internet your computer make a DHCP Discover broadcast and send it your ISP DHCP Server whatever it is and dhcp server replies with DHCP discover/ack and give the ip address in case dhcp server doen't reply or assigned you the ip address you can not go on the internet.
Backdoor is a malicious program which is installed in your machine without your knowledge mostly when you have download free software , games , songs and etc from the internet. Once it installed in your machine by attacker he get the full access in your computer it can also bypass the security or antivirus couldn't detect it. But its not always used to harm your pc but sometimes used to troubleshooting the machine like programmers or system administrator installed backdoor in your machine to troubleshoot your desktop or computer but if attacker get to know you have installed backdoor he will get into your network or harm your network or machine.
