both PCR primers and sequence adaptors are artificial DNA oligonucleotides, generally of known sequence. The first are used to prime DNA replication reactions. Those used to amplify specific DNA sequences in vitro are called PCR primers. Sequence adaptors are any kind of short DNA sequence serving the scope of fishing a (generally unknown) DNA sequence of interest for various purposes; they are used in a variety of techniques, and sometimes can take part in a DNA replication step (e.g. in a 5'RACE). Depending on experimental protocols and following bioinformatics, both PCR primers and sequence adaptors can end up in raw sequence reads (for instance, this may occur if you are sequencing a DNA library).