LaTeX is a collection of TeX macros (= new commands) written "on top of" TeX, in order to simplify TeX's usage. LaTeX's macros are organized in "document classes", e.g. article, book, report, letter, each of which entails definitions pertaining to that particular document style.
Hence, each latex input file should contain, near its beginning, the line
\documentclass{......}
Please note that in a previous version of latex, called LaTeX 2.09, "classes" have been called "styles". The current working latex, which uses classes, is also called LaTeX 2e. If you still use the old notation \documentstyle ..... , latex2e will automatically transfer control to latex2.09 when compiling your latex file.
Apart from class definitions which are bundled together within class files, and which facilitate usage of latex, there exist many package files which are files containing macros pertaining to a certain aspect of a user's latex file; for example including graphics, creating various headers, changing multi-column text, etc. There are hundreds of package files which are available.
When using LaTeX, you specify the class at the beginning of your input file, and then all the necessary arrangement as to headers, sections, table of contents etc are automatically made. You can change the default looks of your file by either using latex keywords (which are words starting with a backslash) or by including a package file in your text and utilizing it.
LaTeX usage is usually easier for beginners then TeX's. However, some experienced users prefer the flexibility of TeX.
Whenever TeX is installed on a computer system, LaTeX is probably installed there also.
For a user guide to latex 2e (The new latex) -- 33 pages -- click here
Please note that the guide is a PostScript file. You will be able to read it (or print it) if you have installed software which displays PostScript (e.g. GhostView).