Fifth declension is the smallest declension - not very many words are part of it. You can tell that a word is fifth declension if it has -ei in genitive singular.

Here are the endings for fifth declension:

​	Singular	Plural
Nominative	-es	-es
Genitive	-ei	-erum
Dative	-ei	-ebus
Accusative	-em	-es
Ablative	-e	-ebus

And here's an example of a fifth declension word declined. For some reason, only two words in fifth declension actually have all of their forms - dies, diei (day) and res, rei (thing/matter). Most fifth declension nouns don't have plural forms. Dies, diei is even more unusual because it is the only masculine word in fifth declension.

​	Singular	Plural
Nominative	dies	dies
Genitive	diei	dierum
Dative	diei	diebus
Accusative	diem	dies
Ablative	die	diebus

Some fifth declension words, like series, -ei and species, -ei have pretty much come straight into English. They became the words series and species, respectively.