It's a complete USB development board that requires no programmers or special cables.

What distinguishes Teensy is its small size and ease of programming and configuration.

Since it has its own on-board programmer and can be connected to PC or Macintosh using the standard USB cable used with most smartphones.

There are many versions available with different processors speeds and memory space.

For example,

Teensy 2.0 is based on ATMEGA 32U4 8 bit AVR Microcontroller with speed of 16Mhz

While

Teensy 3.6 is based on Freescale 32 bit ARM Cortex-M4 processor with 180MHz speed

With full specs of

180 MHz ARM Cortex-M4 with Floating Point Unit
1M Flash, 256K RAM, 4K EEPROM
Microcontroller Chip MK66FX1M0VMD18 (PDF link)
USB High Speed (480 Mbit/sec) Port
2 CAN Bus Ports
32 General Purpose DMA Channels
22 PWM Outputs
4 I2C Ports
11 Touch Sensing Inputs

What makes Teensy special from any other board?

Arduino compatibility makes a great advantage for any development board.

But most Arduino compatible boards come with limited capabilities like speed, memory and peripherals.

That makes the requirement of additional shields essential.

Other non Arduino compatible boards come with many features and connectivity but require special software and tools to learn.

That's what made Teensy Boards special.

Where you have a powerful USB development board fully compatible with Arduino IDE and collaborated previously written code.

Check out the table below for complete family comparison:

	Teensy LC	Teensy 3.2	Teensy 3.5	Teensy 3.6
Clock Speed	48Mhz	96Mhz	120Mhz	180Mhz
RAM	8K	64K	192K	256K
Flash	62K	256K	512K	1M
I/O Pins	27 (all on headers)	34 (24 on headers)	62 (42 on headers)	62 (42 on headers)
Pin Voltage	3.3V	3.3V (5V tolerant)	3.3V (5V tolerant)	3.3V
EEPROM	256 Bytes	2K	4K	4K
Connectivity	USB, Serial, SPI, I2C, I2S	USB, Serial, SPI, I2C, CAN, I2S	USB, CAN, I2C, SPI, Ethernet, SD, I2S	USB (High Speed), CAN, I2C, SPI, Ethernet, SD, I2S
Timers	7	12	14	14
PWM Outputs	10	12	20	20
Analog Inputs	13	21	25	25