The leading strand and lagging strand are the two sides of the original DNA molecule; both are being replicated at the same time. It is easier to replicate the leading strand--this is the one w/the phosphate group at the end (the 5' end.) DNA polymerase can continuously add to the newly created complimentary strand as the original is being unzipped. The lagging strand is the 3' strand--the one w/no phosphate group at the end. Because the additions are essentially being added "backwards", the addition to the new strand requires another step involving RNA primers. The below link illustrates this better than I can describe it! Both strands are copied rapidly, at about 1,000 bases added per second.