A loose sentence expresses the main idea first:Interesting, huh? :)
The girl wrote with incredible monotony, not realising how much it annoyed her readers.
A periodic sentence saves the main statement to the end:
Not realising how much it annoyed her readers, the girl wrote with incredible monotony.
In the loose sentence, the chief contention is clear from the start; in the periodic sentence, you must read to the end for the full meaning. Used too frequently, the periodic sentence can annoy the reader.
I'm very tired. We won at outdoor indoor football tonight. Hooray! Was good to win against that team too, since we drew the last couple of times we played them. Had cannelloni for dinner, with spinach & rocket on the side.
And now, it's study time.
For you, some links: