milestones

Here is a summary of the sort of records you can expect to find. It's my own compilation, and as such I take no responsibility for errors!

pre-1538

The only sources of information available are largely wills and property deeds, etc.
Virtually impossible to trace before this time, unless your ancestors were seriously wealthy or notorious criminals...

No specific format, records should simply be written down.
Births, marriages and deaths are often all mixed together.
Often in Latin - handwriting usually needs a professional palaeographer to translate.
If you find your ancestors in a parish with records back this far, and not destroyed by damp or eaten by mice - it's just possible to trace back this far.

Very few registers kept during this time; baptisms didn't happen, burials were not registered and marriages were before a magistrate.
Many registers before that time were destroyed.
It's difficult to prove a definite link between the period before and the period after this - it's usually a whole missing generation (at least), though if the same surname or family turns up in the registers either side of the gap, you may wish to take an educated guess.

Parish registers begin again.
Difficult to read to modern eyes, and often inconsistent (depending on the incumbent), but often the earliest real records.

Handwriting becomes easier to read from a modern viewpoint.
Inconsistencies continue, but possibilities increase greatly of being able to decipher entries.

Less need to rely on organisation of priest and parish clerk, easier to read.
Still no information about the parents of those being married and buried.
Things are becoming much easier. Especially if your ancestors have been good enough to stay in one place for a long time.
If they moved around - looking for work, etc. - the search becomes much longer, as events could have taken place anywhere within a fairly large radius, and there is still no civil registration to track them down.

It is not yet a legal offence not to register, so many don't.
Addresses in rural areas are often given as a village, rather than a house or street.
Much more detail is available:
- Baptisms: parents' names, place of residence, mother's maiden name
- Marriages: names and professions of fathers; name, age, residence, profession of bride & groom
- Deaths: date, place, cause of death, name and relationship of informant
To obtain this information, however, one needs to either find the original certificate in a County Records Office (if you know which village/town we're looking for), or pay for a certificate if you have the correct reference in the Family History Centre Indices.

Ages are rounded up to the nearest five years.
Sex and profession given.
Place of birth is limited to "whether born in the same county" (as the place of census).
However modest the information available, if you can find the family it will often give a fair idea of the structure of the household - if only by looking at the ages (no relationships are given at this point).

This is much more helpful - precise ages are given (at least, as far as the subjects were willing or able to tell the truth!), and village or town of birth is given, together with relationship to the head of the household.
These are extremely valuable, as entire genealogies can be constructed from them, especially when backed up by parish registers and civil registration.
The censuses can often provide confirmation of whether or not your conclusions are correct about the relationships between individuals featured in parish registers.

... although some still manage to slip through the net!
You had six weeks after a birth to register; paid a fine if registered between 6 weeks & 6 months; after 6 months it couldn't be registered at all (this may lead to inaccurate dates of birth to make registration legal. One of my own family had two birthdays because of precisely this problem!)

... and begin to include additional information:
- the mother's maiden name against a birth
- the surname of the spouse against a marriage
… which makes it much easier to be certain that we have the correct family before paying out for a certificate!