wiki has an entry:

http://en.wikipedia.org/wiki/Doctorate

The title of 'Dr' used to be reserved for people who had earned a 'PhD' degree. Basically... Ones education goes like this:

Bachelors degree (3-4 years depending on which system)
Honours degree (the fourth year in a three year system or earned concurrently with the bachelors in a 4 year system)

You may then apply to do a PhD. It is becoming increasingly common for people to do a Masters degree (One year of coursework or research) before applying for a PhD because it is quite hard to gain entry. If your thesis isn't up to standard sometimes it will be reccomended that you transfer into a MPhil instead of a DPhil degree - especially in the British system.

A PhD then takes either 3 years (thesis only) or 5 years (2 years coursework then 3 years thesis) depending on the system.

You are only allowed to use the titles (BA, BA(hons), MA, Dr etc) after your degree has been 'conferred' in a ceremony (whether you are present or not).

Many people who work in applied fields with patients won't call themselves 'Dr' (even though they are entitled to) because to the general public 'Dr' tends to be equated with 'MD' (a medical degree so one is lisenced to practice medicine). Even though PhD holders were the original doctors and applied professions (including clinical psychology and medicine) were only derivatively associated with the 'doctor' title.

A PhD is a research degree in the first instance. It tells people that one is able to publish to the standard required by the peer reviewed journals in the field. It also lisences one to teach students at university and supervise students for their PhD degrees etc.

Certificates are available for a variety of things... The worth of the certificate depends on a few things... The time taken to earn the certificate (one that requires three years of full time study is clearly worth more than one that required attendance at one seminar). The quality / reputation of the university (or institution more generally) that conferred the certificate. The entry requirements, coursework requirements etc involved in obtaining the certificate (a certificate earned where you need a PhD to enter the course will be worth more than a certificate that has an open entry requirement. A certificate earned where there were hard exams required in order to obtain it will be worth more than a certificate that is attendance based).

A PhD (with the associated 'Dr' title) is the highest academic qualification that can be earned. In order to earn it you need to study for a number of years (between 7 and 9 - and most people take much longer than that). If you get a research PhD then you need to demonstrate a high degree of knowledge of the bredth of the field, and you need to demonstrate a much higher degree of knowledge of a particular area in the field. You also need (with your PhD thesis) to have established yourself as a world leader in a particular (usually very narrow) area. An expert. THE expert (in your particular narrow area).

It used to be that PhD's were about research. Research was what was meant to distinguish universities from other tertiary instiutions. Giving PhD degrees to people not engaged in research (e.g., Lawyers, clinical psychologists etc) was controversial. Allowing other tertiary instiutions (e.g., polytechnics) to confer bachelors degrees was also controversial.

I guess what happened was that PhD and degrees more generally were taken to be associated with a 'high standing' qualification. ANd clearly we wanted to give those to applied practitioners of respectable fields (such as physicians, nurses, lawyers, dentists) etc too.

Urgh.