A.D. (Anno Domini, year of the Lord) and B.C. (Before Christ) have long been used by archaeologists in Europe and the Americas; their reference point is the birth of Christ.
C.E. (Current Era) and B.C.E. (Before Current Era) are sometimes used instead of A.D. and B.C. as a way to express a date without specifically referencing Christianity. But the dates themselves are exactly comparable to dates expressed as A.D. or B.C. For example, 1280 C.E. is the same year as A.D. 1280, and 228 B.C.E. is the same year as 228 B.C.
B.P. (Before the Present) is the number of years before the present. Because the present changes every year, archaeologists, by convention, use A.D. 1950 as their reference. So, 2000 B.P. is the equivalent of 50 B.C.
ka (kilo annum, thousand years) signifies "thousand calendar years ago," and it is used most often in geological, paleontological, and archaeological reporting to assign a general date to events that occurred a very long time ago. For example, the entry of humans into the New World during the Pleistocene (Ice Age) is thought to have occurred by about 15,000 years ago, or 15 ka (which is equivalent to approximately 13,000 B.C).