It is a large piece of manufactured metal specifically designed to protect the front of a vehicle from collisions that can damage the engine and all of its components. Bull bars are generally used by those who tour long distances as they will protect against animal strikes and it's also commonly used by those who are off-road as a bull bar will help protect their 4WD from debris.
This answer will vary as it will depend on the location you will be travelling to and what you will be using your bull bar for. Steel is a very popular option, and most people will choose this material because of its strength. However, aluminum is another excellent option you can consider if you are only going to be venturing off the road or into animal strike areas infrequently, as this material would be very efficient.
Bull bars are designed to keep them as close to the vehicle and as high as possible and will help increase your approach angles while increasing "off-roadability" and providing your tires with more clearance so that your vehicle can climb over obstacles. If, however, you don't plan on going off-road and will be touring on rural highways, you can choose a bull bar that is designed to hang lower as this may be more beneficial.
Depending on the vehicle, you may notice an extra drop in the front suspension, and your front end will feel softer. Handling will be worse, as will braking, so a suspension upgrade is recommended as this is the only way your suspension will be able to handle the added weight.
Nothing major will have to be moved, but small components like your windscreen washer bottle, for example, may need to be moved. The model of your vehicle will dictate whether extra time is necessary to relocate specific engine components. It is very rare for a modern bull bar not to work with a vehicle's electronics.
Very little care is needed with steel bull bars. If you ever notice that the powder coat has begun to rub off from bumps and scrapes as a result of hitting trees and rocks, touch it up as soon as possible so that you don't experience further damage. Polished aluminum bull bars can stain over time. However, they can be re-polished, and a professional cleaning every few months is highly recommended as this will help keep your finish in good condition.
Depending on the severity, you may require a complete replacement or just a partial one. Take it back to where you purchased your bull bar, or take pictures and send them to the manufacturer to receive an accurate answer.