What causes residual sweetness

[p_p]

I understand that a high FG does not necessarily mean that beer will taste sweet; dextrin malts, wheat, oats, etc. will result in higher FGs without extra sweetness while using other speciality malts such as crystal will result in residual sweetness.

What are the malts that contribute to sweet beer and what kind of sweet-tasting, unfermentable sugars are contributed by these speciality malts?

Thanks,
p_p