Explain this .htaccess snippet - regex

Question

Can someone explain the following htacess lines, I understand parts, but would like a deeper knowledge. As a note I assumes it works as intended, this is not currently live, I am just reading through some workbooks and this was printed.
// Don't understand this line
Options -Multiviews
// Don't understand this line
Options +FollowSymLinks
// Understand this line
RewriteEngine On
// Don't ~fully~ understand this line, esp. in context
RewriteBase /portfolio
// Don't ~fully~ understand this line
// I understand that its asking if the filename is a valid file or dir
// but is it overall saying if valid file or valid dir perform rewrite?
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
// Don't understand, $1 is the STRING, and the rest the condition, but
// how do you read this condition?
RewriteCond $1 !^(index\.php|images|robots\.txt|css)
// Don't understand (but do understand the RewriteRule PATTERN REPLACE, but is it
// saying replace 'all' with index.php/all ?
RewriteRule ^(.*)$ index.php?/$1

Answer 1

Options -Multiviews
This disables the Multiviews Apache option. Basically, the option allows the server to look for content in the same directory using different file names based on the content types and languages accepted by the client. The directive is just disabled in this case to make sure Apache doesn't serve any unexpected files.
Multiviews enables content negotiation, which is explained at: http://httpd.apache.org/docs/current/content-negotiation.html
Options +FollowSymLinks
This makes sure the FollowSymLinks option is enabled. This setting allows Apache to follow symbolic file links in the directory if they exist. This setting exists in case there are symbolic file links to make files physically exist elsewhere on the server than what is requested.
Longer explanation at: http://www.maxi-pedia.com/FollowSymLinks
RewriteBase /portfolio
This setting is for defining the base path for the url used by the rewrite engine. When the rewrite engine rewrites the url in .htaccess, it strips away the path to the current directory. Once the url rewriting is complete, it will add it back based on the current file directory. However, sometimes the url that is requested does not have the same path as the directory structure on the server itself. The RewriteBase tells the rewritengine what the URL path is to the current directory. In this case, for example, the files may be stored in /foo/bar, but they are accessed via the browser as www.example.com/portfolio. The RewriteBase tells the engine to add /portfolio to the url, instead of /foo/bar.
For complete explanation, see: http://httpd.apache.org/docs/current/mod/mod_rewrite.html#rewritebase (the url also contains explanations to the other Rewrite parts of the .htaccess).
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
These lines make sure that any url that is an actual existing file or directory wont get rewritten. The ! before the condition is negation. So these two conditions should be read as ifNotFile AND ifNotDirectory.
RewriteCond $1 !^(index\.php|images|robots\.txt|css)
The $1 here refers to the sub pattern capture 1 of the actual rewrite. In other words, it means the part captured by (.*) in the RewriteRule. Basically, this rule simply checks that the RewriteRule wont rewrite any url that starts with "index.php", "images", "robots.txt" or "css".
RewriteRule ^(.*)$ index.php?/$1
This simply tells the rewrite engine that any request (that isn't prevented by the rewrite conditions, of course) should be rewritten to index.php? with the actual request following it. Just like you said, a request foo/bar will be forwarded to index.php?foo/bar. The point is to allow index.php to handle the file requests (which can access them via $_SERVER['QUERY_STRING']), which is very common practice in CMS systems and frameworks.
I hope these explanations will help. I don't have extensive experience on all these directives, so slight inaccuracies may exist, please comment if so.