Copyright infringement arises from re-using someone’s work without the permission of the copyright owner (or the benefit of a licence or suitable copyright exception) and is a legal issue. On the other hand, plagiarism arises from re-using someone else’s work in a way which implies it is your own. It is essentially a matter of ethics and academic discipline rather than a legal issue, although the consequences can be very serious.
Naturally the two problems can often overlap (re-using work without a legal basis and without acknowledgement). The key to avoiding plagiarism is always to acknowledge other people’s work when you are quoting form it or when relying upon ideas developed by someone else. That way you can’t be suspected of passing it off as your own work.
The UCL Copyright team are often asked about the dangers of self-plagiarism:
“Will I be in danger of self-plagiarism if I re-use material from my thesis in a published article?” or conversely perhaps: “Can I use material from my previously journal articles in my thesis?”
Self-plagiarism is a real issue, in the sense of recycling you previous work as though it were wholly original, in a context where a certain level of originality is essential. The key to avoiding this danger is very similar to avoiding any kind of plagiarism: You need to be scrupulous about citing your own previous work where you are quoting from it or relying upon it.
In the context of your thesis there may be separate academic issues about relying too heavily on your previously published work even though you are crediting it scrupulously, so in those circumstances it would be good to discuss that with your PhD supervisor at an early stage.