Before the Norman Conquest of Britain, people did not have hereditary surnames: they were known just by a personal name or nickname. Saxon communities were quite small - each person was identifiable by a single name, but as the population increased, it gradually became necessary to identify people further. These additional names were 'bynames' used to describe one individual for reasons of better identification. The first location names were used up fast, so additional descriptions arose from nicknames, physical attributes, counties, trades, and later even devices used as heraldic charges were used.
Most Saxon personal names - names such Oslaf, Oslac, Oswald, Oswin and Osway ('Os' meaning God) - disappeared quite quickly after the Norman invasion. It was not fashionable, and possibly not sensible either, to bear them during those times, so they fell out of use and were not often passed on as surnames. However, some names from before the Norman Conquest survived long enough to be inherited directly as surnames, including the Anglo-Saxon Cobbald (famous-bold).
Surnames were introduced into England by the Normans after the conquest in 1066. They were used for law and tax reasons when describing land. and the practice gradually spread. Initially, the identifying names were changed or dropped at will, but eventually they began to stick and to get passed on.
Thus the concept of a name handed down from father to son start to appear sometime between 1350 and 1450. This trend lead to bynames such as John the butcher, William the short, Henry from Sutton, Mary of the wood, Roger son of Richard, John the Smithy, Peter the arrow maker (Peter Fletcher) being adopted as the family name for the children of that individual. Over time many names became corrupted and their original meaning is now not easily seen.
New surnames continued to be formed long after 1400, and immigrants brought in new ones. Many Irish and Highland Scottish names derive from Gaelic personal names, as do those of the Welsh, who only began to adopt the English system of surnames following the union of the two countries in 1536. This is all too far back to be helpful in researching family origins, although the study of a particular surname may be useful when the investigation points to an area where it appears often.
Another complication is that sometimes two different names can appear to be the same one, being similar in sound, but different in origin. The fairly common name of Collins is an example of this. It comes from an Irish clan name, but it is also one of several English surnames derived from the personal name Nicolas.
Thus you can see that only by tracing a particular family line, possibly back to the 14th century or beyond, will you discover which version of a surname is yours. It is more important to be aware that both surnames and forenames are subject to variations in spelling, and not only in the distant past. Standardised spelling did not really arrive until the 19th century, and even in the present day variations occur, often by accident - how much of your post has your name spelt incorrectly?
If we have problems today spelling your name, then have a heart for our ancestors. For the most part, a large portion of medieval Europe was illiterate, including the upper classes, and the records of names kept were maintained by priests or clerks. The spelling was thus recorded as what the priest thought he heard during a marriage, baptism or burial, and his own ideas about spelling were used.
Before the reformation, as far as spelling went, most records were in Latin, and there is a period of great confusion when English appears around the 1540's, which is not fixed until 1670 into any conformity. But this only fixed the everyday words, names were down to local dialect.
Also the English civil wars leave a large black hole in the records, and most accurate parish records start around 1661, or would have done so if the civil war and restoration had not introduced such debt and religious restrictions to the country, that the king imposed taxes on couples. Many marriages were performed by non-conformist churches, and the system is a mess until 1837 - when central records commence.