Text copied from programs such as Microsoft Word carry formatting with them. If you paste the text into an RTE, the RTE will attempt to keep the formatting as close as to what you had in the previous program.
Sometimes this isn't desirable. When it does happen, you will need to clean the text.
There are two ways to clean text. One is to first paste the text into notepad, then copy the text again from notepad. This will remove all formatting. This option is easy and quick.
The second way to remove formatting is to paste the text into the RTE, highlight all of the text, and then click the 'Remove formatting' button. See below for the an image of where to find the button
From here, make sure you tick at least "MS Word Format". If you copied the text from another website, you can tick "HTML Format". Ensure you click okay and then save.