Both can mean "adj: certain, sure / adv: certainly, surely", but "sicher" is also used to mean "adj: safe, reliable" or "adv: safely, reliably, self-confident". Depending on the context they are interchangeable.

Both have the same meanings in English" certain,or certainly"but there is no adj-endchange for "sicher".Let me give you some examples: sich sicher fühlen(to feel confident) sicher sein (to be certain) sicher eintretendes Ereignis(difinite event) unter gewissen Bedingungen(subject to certain conditions）ein gewisses Risiko(a certain amount of risk) There is some difference between them,so through some examples you can compare their usages,tell the difference and understand it therefore completely.^^Viel Gruß.