A battery booster is just another name for a jump kit - so if your car's battery has gone flat, you can jump it with a jump starter without needing another car to jump-start the battery. A charger, sometimes called a conditioner, keeps the battery charging and maintained if you don't drive the car very often. It means the battery will stay in good condition even if you only drive it every few days/weeks. For most people, I'd recommend having a jump starter like this one (but we haven't personally tested this model): amzn.to/31MQreY

It means that you can jump-start the battery if it goes flat without needing to call recovery or get another driver to use their car to jump yours. Whereas, if you drive regularly and the battery keeps dying, I'd suggest getting the battery itself repaced at Halfords or the like. I only recommend battery conditioners to those who find they keep getting a flat battery due to low mileage. Chargers, like those made by CTEK, need a mains power supply too so it only works if you have a garage with mains power available. Hope that helps.