Trinket may be small, but do not be fooled by its size! It's a tiny micro-controller board, built around the Atmel ATtiny85, a little chip with a lot of power.
There are two versions of the Trinket. One is 3V3 and one is 5V. Both work the same, but have different operating logic voltages.
Use the 3V3 one to interface with sensors and devices that need 3V3 logic, when you want to power it off of a LiPo battery, or if you want to connect it directly to the Raspberry Pi GPIO without logic level converters. The 3V3 version should only run at 8 MHz. As of May 27th, 2015 the 3.3V Trinket has been revised! The board is now even smaller - at just 27mm x 15mm - and comes with a micro-B USB connector rather than mini-B
Use the 5V one for sensors and components that can use or require 5V logic. The 5V version can run at 8 MHz or at 16MHz by setting the software-set clock frequency. As of October 9th, 2015 the 5V Trinket comes with a micro-USB connector instead of a mini-USB connector!
Adafruit wanted to design a micro-controller board that was small enough to fit into any project, and low cost enough to use without hesitation. Perfect for when you don't want to give up your expensive dev-board and you aren't willing to take apart the project you worked so hard to design. It's their lowest-cost arduino-IDE programmable board!
The Attiny85 is a fun processor because despite being so small, it has 8K of flash, and 5 I/O pins, including analog inputs and PWM 'analog' outputs. Adafruit designed a USB boot-loader so you can plug it into any computer and reprogram it over a USB port just like an Arduino. In fact they even made some simple modifications to the Arduino IDE so that it works like a mini-Arduino board. You can't stack a big shield on it but for many small & simple projects the Trinket will be your go-to platform.
Here are some useful specifications!
ATtiny85 on-board, 8K of flash, 512 byte of SRAM, 512 bytes of EEPROM
USB boot-loader with a nice LED indicator looks just like a USBtinyISP so you can program it with AVRdude(with a simple config modification) and/or the Arduino IDE (with a few simple config modifications)
Mini-USB jack for power and/or USB uploading, you can put it in a box or tape it up and use any USB cable for when you want to reprogram.
Adafruit really worked hard on the boot-loader process to make it rugged and foolproof, this board wont up and die on you in the middle of a project!
~5.25K bytes available for use (2.75K taken for the boot-loader)
On-board 3.3V or 5V power regulator with 150mA output capability and ultra-low dropout. Up to 16V input, reverse-polarity protection, thermal and current-limit protection.
Power with either USB or external output (such as a battery) - it'll automatically switch over
On-board green power LED and red pin #1 LED
Reset button for entering the boot-loader or restarting the program. No need to unplug/replug the board every time you want to reset or update!
5 GPIO - 2 shared with the USB interface. The 3 independent IO pins have 1 analog input and 2 PWM output as well. The 2 shared IO pins have 2 more analog inputs and one more PWM output.
Hardware I2C / SPI capability for breakout & sensor interfacing.
Works with many basic Arduino libraries including Adafruit Neopixel!
Mounting holes! Yeah!
Really really small
Limor has put together a great introduction to Trinket which goes into a lot of detail including examples, pin outs, and more: check out the introducing Trinket tutorial
