These are sent out regarding the decision on your application, and will include your application reference number, Home Office reference number, and they may or may not be signed. They follow standard formats, with just your reference number and particulars about your case being unique.
In the case of decision letters, there will be a short covering document, and then a more substantial, numerated decision document (usually signed at the end). For those familiar with such ‘decision & reasons’ documents the phenomenon of stock blocks of copy & paste text will be readily apparent, and stand out. The UKVI provides its ECOs and its caseworkers with assessment guides in conjunction with which they can access such stock response material. Look for the theme of stated consideration of vital supportive evidence you provided with your application, but actual evidence that these hadn’t been considered at all or only in a skimmed/superficial level way – errors on facts, dates, names, etc. of importance are also common features in these letters.
If your application is refused you will be provided with a further, third document; an appeal request form (with a deadline for use of usually 14 days from the date of the decision: on deadlines please visit http://needtoknow-immigrationuk.com/appeal-deadlines/
Be aware that very commonly these UKVI refusal document package will receive No note on or any reference to your passport (this applies in the UK only, with UKVI caseworkers, and Not ECOs overseas – you will often get your passport back with the free gift of a permanent ‘black mark’ refused signal THAT WAS NEVER REFERRED TO AS TO BE GIVEN when you submitted your application) being potentially seized with no request permission or explanation of why it has been taken. You may have to write to the UKVI about why they did this and didn’t even have the politeness to inform why – pure intimidating, bullying Hostile Environment ‘customer care.’