Common Law is law that's based on precedent. That is, law based on court rulings or case law rather than on written legislation passed through parliament. Common Law is based in the idea that if the facts of cases are similar, it is only fair that the outcomes or rulings in the cases should be similar as well.
This means that the decisions made by judges and juries have implications beyond the cases they are ruling on, because they can affect the outcomes of future cases as well.
Every lawyer should have a solid knowledge of common law, especially when it relates to their client's cases.
Common Law can also refer to marriage or partnership, and is sometimes referred to as de facto. A Common Law Marriage is recognised in some jurisdictions when two people have been in an interdependent partnership for a certain number of years, and even though no official marriage ceremony has taken place, the partnership is recognised on the same level as a legal marriage.
If a common law partnership breaks up and the couple have shared assets or children together, the break up may be similar to a divorce, and a lawyer may be required.