They are equivalent, but one does not count as the other. You can fly anywhere in the world using an ATP or an ATPL, but you must possess the license required by the country in which your airline is based. So if you work for a US airline, an ATP is good enough for international flying. But if you want to work for a foreign airline, you will probably need an ATPL (some foreign airlines will accept an FAA ATP)

The ATPL is the JAA (european) version of the ATP. Other countries use the the term ATPL also. The training standards and ground school required for an ATPL are much more stringent than for an FAA ATP...many holders of a US ATP would never be able to pass the written exams for an ATPL. The flight experience requirements are very similar though.

A frozen ATPL means that you have passed all the tests, but do not have enough hours to hold the ATPL. Once you acquire the necessary flight hours, they will convert the frozen ATPL into a regular ATPL. Most or all european airlines require at least a frozen ATPL. There is no US equivalent...in the US, you cannot take the checkride until you meet the experience requirements.

If you need to convert your US ATP to an ATPL, expect to spend six months and thousands of dollars.