In DNA replication both the parent strands function as template strands.
On the template strand with 3' → 5' polarity, the new strand is synthesised as a continuous strand. The DNA polymerase can carryout polymerisation of the nucleotides only in 5' → 3' direction.
This is called continuous synthesis and the strand is called leading strand. On the other template strand with 5' → 3' polarity, the new strand is synthesised from the point of replication fork, also in 5' → 3' direction. But, in short fragments, they are later joined by DNA ligases to form a strand called lagging strand.