Peripheral Interface microControllers are small programmable computers that are used in circuitry to control various sensors and peripherals. The term I grew up with was PIC, but this has since been replaced by neologisms like “Arduino“, which is colloquially used to describe microcontrollers generally, but more accurately describes specific chips you can program with the open source Arduino Integrated Development Environment or chips made by the Arduino foundation.
I am a fan of the AVR series of 32-bit and 8-bit controllers from Amtel, particularly the ATmega328P and Attiny85.
I buy all my electronics on ebay which is the only way that I can handle the cost of this hobby. For the Mega328 chips I usually purchase Arduino Nano clones. These are great little boards and cost $3 a piece if you are willing to wait a little bit on shipping from China. For ~$1.50 you can grab an 8pin Attiny85 chip, but you need an external programmer to program these. I simply use an Arduino Nano as an in serial programmer.
With microcontrollers I make all sorts of electronics and robots. Mostly I want to learn engineering and programming, and so the Arduino platform makes it easy for me to dip my feet in. From what I have read, the IDE language distances the user from the machine code and so it is recommended that users learn to implement and understand machine code, if nothing else, to understand what is working “under the hood” of higher level coding languages.