If you already know the basics of search operators, the cheat sheet is here
rule34.xxx/index.php?page=help&topic=cheatsheet
have fun

But if you're used to algorithmic garbage and don't know what all that stuff on the cheat sheet means, I'm about to blow your mind.

When you enter something in the search bar, it turns up those exact tags. Type "boobs" and you get every post tagged "boobs". That's all. This is not the same as every post that has tags about boobs, though, and you'd have to search for "breasts" and "tits" separately if you wanted to be sure to see all the boob posts on the site.
This is where search operators come in.

The space functions as an AND search. Entering "boobs short_hair" will find you every post that has the both the "boobs" tag and also the "short_hair" tag. rule34.xxx/index.php?page...mp;tags=boobs+short_hair+
It will not show you posts with only one of those tags.
Note the underscore in "short_hair", that's how the site handles tags that contain more than one word without accidentally turning them into AND searches. searching "short hair" without the underscore would turn up every post tagged with both "short" and "hair" as separate tags.

The tilde ~ is the new OR search, and your new best friend.
You enter "boobs ~ breasts ~ tits" into the search bar, and you get a page with every post tagged with any of those tags. It's essentially three searches in one. It's going to make your life better.
rule34.xxx/index.php?page...oobs+%7e+breasts+%7e+tits

And to get really fancy, you can combine operators.
Typing "short_hair ( boobs ~ breasts ~ tits )" will show every post tagged "short_hair" that also has one of the three tags in the parentheses. The parenthesis are important and work just like order or operations in arithmetic, if you remember that from school. If not, just think of it as a bucket, you're looking for the stuff outside of the bucket that also has at least one tag from inside the bucket.
rule34.xxx/index.php?page...+%7e+breasts+%7e+tits+%29

You can have multiple buckets, too.
"( black_hair ~ blonde_hair ~ pink_hair ) ( boobs ~ breasts ~ tits )" will show you posts that have something from bucket A and something from bucket B. We have two buckets full of OR searches, and the space between them is an AND search
rule34.xxx/index.php?page...+%7e+breasts+%7e+tits+%29
You will see posts tagged "blonde_hair" and "tits" alongside posts taged "black hair" and "boobs". This is great for when you want to broaden a search or when you're searching for something very specific that could go by a lot of different words.

You can also filter something out of a search with the negative sign -
"( boobs ~ breasts ~ tits ) -pink_hair" will show you the results of our combined bosomy search, but remove any post tagged "pink_hair". It's like a temporary blacklist for something you don't want in this search but don't want to nuke entirely forever.
rule34.xxx/index.php?page...+%7e+tits+%29++-pink_hair

The asterisk * is a wildcard. It will find any tag that contains the letters you entered and then any other letters at all whereever you put the asterisk.
Searching for "*girl" will show us any tag that ends with "girl", including "catgirl", "1boy1girl", and "tanned_girl". It wont show tags that contain "girl" in the middle, like "1girls", or tags that start with "girl", like "girl_on_top". It will include "girl_on_girl", since that ends with "girl".
rule34.xxx/index.php?page=post&s=list&tags=%2agirl

The cheatsheet linked at the top has a few more, but AND, OR, NOT and the wildcard are the main ones for finding the porn you want.
Happy fapping.

Another hot tip: if something you're into has multiple tags it goes by, you can bookmark an OR search for all those tags and then just fill in specifics without having to type out the whole thing.
I've bookmarked a "gay shit" OR search, so I can just slap this link from my browser and then type in femboy or bara or huge_cock or BDSM or whatever I want the gay version of at the moment.

rule34.xxx/index.php?page...2fmale+%7e+males_only+%29

Actually, I've found combining the negative with an "OR" group doesn't work that well.

The OR functionality is brand new and they're still working out some glitches, but in general it's working fine for me. For an example that should be obvious at a glance, compare my Gay Shit OR group
rule34.xxx/index.php?page...2fmale+%7e+males_only+%29
to (gay shit) -furry -anthro rule34.xxx/index.php?page...s_only+%29+-furry+-anthro

Any furry art in the second search is simply untagged.

oh, so you CAN do such searches. thanks. on twitter you can do ti by doing (word1 OR word2 OR word3)