An environment is a command that lets you manage a segment of text differently than you set the variables for the rest of your document. For example, you create a report with font size 12, but you need to change your font size for a few sentences. The commands \begin{environment}, \huge and \end{environment} will let you temporarily alter the text inside the environment commands to be the
size HUGE.
So, changes are only effective from /begin{environment} blah blah blah change change change \end{environment}. There are no limits as to how make changes you can make inside an environment.
There are many features you can add to your document that will make it more readable and user friendly. Features such as fonts, bold, italics, underline etc. are features you can add to your document and the commands that will make your document look wonderful, but will end with either an \end command or the end of your environment.
\begin{emph} or \begin{em} : This command makes text italicized and this command is valid until the code comes across a \end{em}, \end{emph} or environment. To italicize one word in a sentence, you use the syntax: This is \em{my} sentence; which gives you the output, This is my sentence.
\bfseries{I'm making this text inside the brackets bold} : This command sets your text to I'm making this text inside the brackets bold. The bold command will continue until a close bracket is found.
\qoute : To create a quote inside your document, begin your quote with \begin{quote} and end with \end{quote}.
\center: Centers the text.
\verse : Creates offset text for poems.
\itemize : Makes an itemized list.