The ATtiny43 is a small AVR microcontroller manufactured by Microchip (formerly Atmel). It features an on-chip boost converter, which allows it to run from a single alkaline battery (ex, a basic AA battery) with just an external diode and inductor. This breakout board integrates those components, making this the ideal board for cases where you only have a single AA (or even smaller) to power your project. In fact, if your project only gets to have a single alkaline battery for power, I'm not aware of any other Arduino-compatible part that can do it without an external (and hence battery life destroying) boost converter.

The boost converter is the crown jewel of this part, and the only reason to use this part: If you need to run from a single, partially discharged alkaline battery, and you need passable battery life, you don't really have any other options among AVRs. The current in "Active low power mode" where the boost converter relaxes control over the power rail, just keeping it from falling below the minimum 1.8V required for operation, typical current usage for the whole device is given as approx 5 uA, and 5 mA while running the chip. Those are very difficult specs for an external boost converter to beat. The one I happened to have on hand, fairly commonly available from chinese vendors did 70uA no load, and supplying just a 3mA load, that jumped to over 16mA, doesn't even compare. Even the best spec'ed ones I've seen claim around 13uA, and I'll believe that spec when I got the one I ordered and measure it - but the power consumption of the integrated boost still puts that one to shame.

Basically, if you need to power an AVR, on a single alkaline battery, and need decent battery life, this is the only option. The battery life is much worse than if you had two alkaline batteries of half the capacity, in series to get ~3v, so you could run them down to 0.9V (by which time the series voltage would be under 1.8) and used basically any AVR and proper power management practices. If you only have space for a single cell, you often can't get any other options that are the same, just half the length (these exist, but are exotic), or you're prototyping something that will be used by consumers who need to be able to buy replacement batteries easily, this is your best option. The rest of the featureset is a bit meagre - but I suspect the reason for that was an overarching effort to limit power consumption of the chip, sine every mA drawn at 3v, is over 3 mA from the battery's perspective. (yes, this implies that the CPU core in active mode is using under 2mA when the spec sheet says 5mA drawn at vBat for active mode with boost conv in active regulated mode

Arduino support is provided by ATTinyCore which I maintain.

Boost converter capability

The boost converter will start up as long as the battery voltage is 1.2v or higher (in my testing, it seems to start at 1.1v), generating 3v. It will keep running as long as VBat is at least 0.8v, possibly even lower. It is capable of supplying 30mA to external devices as long as VBat > 1.0v (at lower VBat, the maximum current is lower - if overloaded, 3v output is not guaranteed)

ATtinyCore for Arduino for ATtiny43 features

• Universal Wire.h and SPI.h libraries included with core will transparently use the USI for interfacing with SPI and I2C devices. Normal libraries for these parts can be used without modification to work with the ATtiny.
• Built-in Software Serial port named "Serial" (for code compatibility) on pins AIN0 and AIN1. This is still a software implementation, so it is half-duplex (you can't receive and send at the same time), and the baud rates should be kept low. It uses the ACO interrupt, instead of PCINTs, so that PCINTs can be used for other purposes.