An Arduino is kind of a microcontroller motherboard. A microcontroller is a simple computer that can (generally) run one program at a time and very simple to use. A microcontroller is a single chip that contains a CPU, memory and programmable input/output peripherals. Arduino Uno is based on an 8-bit AVR microcontroller. Other Arduino boards use different microcontrollers, some of them a lot more powerful than the one used in the Arduino Uno.
A Raspberry Pi is a low power and small size full computer. It is a general purpose computer. A Raspberry Pi is based on a 32-bit microprocessor (Raspberry Pi 1 and original Raspberry Pi 2) or 64-bit microprocessor (Raspberry Pi 3 and more recent Raspberry Pi 2), it can run a full size operative system (such as Linux), and by doing so it can run multiple programs at a time. By the way, the microprocessors used on the Raspberry Pi boards are based on the ARM architecture.
Bonus 1
AVR is a family of 8-bit microcontrollers developed by Atmel (now owned by Microchip). ATmega is one family of AVR microcontrollers. For example, the Arduino Uno uses an ATmega328P microcontroller.
Bonus 3
I don't get your "direction" question. Are you trying to choose between a using an Arduino or a Raspberry Pi for your projects? If so, you should try several projects with both, eventually you'll be able to see the best one for every project. Sometimes an Arduino, sometimes a Raspberry Pi, sometimes both.
An Arduino is kind of a microcontroller motherboard. A microcontroller is a simple computer that can (generally) run one program at a time and very simple to use. A microcontroller is a single chip that contains a CPU, memory and programmable input/output peripherals. Arduino Uno is based on an 8-bit AVR microcontroller. Other Arduino boards use different microcontrollers, some of them a lot more powerful than the one used in the Arduino Uno.
A Raspberry Pi is a low power and small size full computer. It is a general purpose computer. A Raspberry Pi is based on a 32-bit microprocessor (Raspberry Pi 1 and original Raspberry Pi 2) or 64-bit microprocessor (Raspberry Pi 3 and more recent Raspberry Pi 2), it can run a full size operative system (such as Linux), and by doing so it can run multiple programs at a time. By the way, the microprocessors used on the Raspberry Pi boards are based on the ARM architecture.
Bonus 1
AVR is a family of 8-bit microcontrollers developed by Atmel (now owned by Microchip). ATmega is one family of AVR microcontrollers. For example, the Arduino Uno uses an ATmega328P microcontroller.
Bonus 3
I don't get your "direction" question. Are you trying to choose between a using an Arduino or a Raspberry Pi for your projects? If so, you should try several projects with both, eventually you'll be able to see the best one for every project. Sometimes an Arduino, sometimes a Raspberry Pi, sometimes both.
Bonus 4
You could try the chipKIT boards: chipkit.net/wiki/index.php?title=B.... These are Arduino-like boards based on PIC32 microcontrollers from Microchip, these microcontrollers use a MIPS32 processor (en.wikipedia.org/wiki/PIC_microcon...). So, even if they're mostly programmed in C (or C++), you can also program them in assembly (e. g. johnloomis.org/microchip/pic32/res...)