The Teensy is a breadboard-friendly development board with loads of features in a, well, teensy package. Each Teensy 3.5 comes pre-flashed with a bootloader so you can program it using the on-board USB connection: no external programmer needed! You can program for the Teensy in your favorite program editor using C or you can install the Teensyduino add-on for the Arduino IDE and write Arduino sketches for it!
The processor on the Teensy also has access to the USB and can emulate any kind of USB device you need it to be, making it great for USB-MIDI and other HID projects. The 32-bit, 120MHz processor brings a few other features to the table as well, such as multiple channels of Direct Memory Access, several high-resolution ADCs and even an I2S digital audio interface! There are also four separate interval timers, plus a delay timer! Oh yeah, and all digital pins have interrupt capability and are 5V tolerant.
All of this functionality is jammed into a 62.3mm x 18.0mm board with all headers on a 0.1" grid so you can slap it on a breadboard and get to work! The Teensy 3.5 (as well as its sibling, the Teensy 3.6) is larger, faster and capable of more projects, especially with its onboard micro SD card port. An upgraded ARM Cortex MCU (120MHz from 72MHz), more memory (512K from 256K), as well as more RAM, EEPROM and accessible pins make up the key new features of this "teensy" board. The Teensy 3.5 is slightly scaled down from the Teensy 3.6 but is offered at a cheaper price point, comparatively.
120MHz ARM Cortex-M4 with Floating Point Unit
512K Flash, 192K RAM, 4K EEPROM
Microcontroller Chip MK64FX512VMD12
1 CAN Bus Port
16 General Purpose DMA Channels
5V Tolerance on All Digital I/O Pins
62 I/O Pins (42 breadboard friendly)
25 Analog Inputs to 2 ADCs with 13-bit resolution
2 Analog Outputs (DACs) with 12-bit resolution
20 PWM Outputs (Teensy 3.6 has 22 PWM)
USB Full Speed (12Mbit/sec) Port
Ethernet mac, capable of full 100Mbit/sec speed
Native (4-bit SDIO) micro SD card port
I2S Audio Port, 4-Channel Digital Audio Input & Output