I'll try and keep this short, but I've met much more experienced players than me who don't know these little tricks to help make scroll browsing a lot easier, so I'll explain all the filters, starting with the basics.
All of the below filters work in the deck builder, in trade (for both you and your partner), crafting, and any other place where there's a search bar in scrolls.
This may be obvious, but I want to cover everything. If you don't put any filters in, whatever you type in, it will search by name, simple enough, if you type in 'pother' you'll get the pother spell. There are a few things to know about this, not that they will be of great use, but I'll cover them anyway. Firstly, you don't have to type it in exactly, you could type in 'raider gravelock' and still get a gravelock raider, or you could type in just 'forge' to get an automata forge (and 'forge' would actually narrow it down more than 'autom'). you can also type stuff from the middle of names, it really doesn't matter. You could type in 'a b c d e' and find out that Death Cap Berserk' is the only scroll with all 5 letters in the title, it is a very flexible system.
typing in t: will filter out the subtypes of all scrolls, from main type (creature, structure, spell or enchantment) to subtypes. subtypes have little effect on gameplay, although they matter. t:human will show you all human creatures, and t:displacement will show you all displacement spells. These have an effect in gameplay with bloodline taint, but certain subtypes get certain buffs, for example, typing in t:automaton will filter out all automatons, so you can see everything that the scout automaton will effect, or t:undead will show creatures and structures that Nuru's dominion will effect, or t:structure for the scrolls that have synergy with machine divinator. This can be helpful for building decks around certain scrolls or themes.
typing d: will filter through the descriptions of cards, you can use this to do a wide selection of things, such as typing in d:draw and finding all the scrolls that draw scrolls, or type in d:relentless to find all relentless creatures (this doesn't just filter attributes, d:relentless will find machination mindset and fierce tactics too, although you can eliminate them with t:creature)
with r: you can filter out the rarity cards. There are visual ways to tell, like the rarer a card is, the more damaged its edges are, and the bottom upturned corners have different markings on the back, but if you have trouble with that, r: can help you. there's only 3 different filters for this, as you'd expect, r:rare, r:uncommon and r:common, however most people just type the first letter, as r:r, r:u and r:c work too. This is ESPECIALLY useful in trade, as you can look through either your own, or your partners rare's with r:r, as usually that's all you'll want to trade, and this can be a useful filter when combined with others.
set filters filter out the set a scroll is in, which for the most part means when they where released. as of 15/03/14 there's 5 sets, so typing in s:5 will filter out all the new cards that you/someone else has, and is helpful for finishing you're collection after each update. Also in trade, newer cards are generally worth more than old ones, so you can find what's worth more via that. You can also use + or - with this filter, to check all the old cards with s:4-, or all the cards since a given update.
ap stands for attack points, and can be used to filter out specific attack values, or with using ap:5+, you could filter out all units which attack for 5 damage or more, or with ap:2- you could see everything underdog's spite effects, or ap:1- for everything that works with unforeseen onslaught.
similar to the attack filter, hp stands for hit points, or health points, and is good to find scrolls that survive certain attacks. hp:1 lets you find anything that would die to kabonk or frost gale, hp:3+ is good for finding scrolls that can survive thunder surge, quake, spark, and other attacks, hp:4+ can find things that can stand up against kinfolk veterans, burn, poison, and many other more powerful 3 damage attacks. using both hp: and d:armor can be good for finding defensive units.
cd obviously stands for countdown, and is used to filter out the 3rd and final main stat. Most units have 2 countdown, although you can find 1 countdown units with cd:1, slower, 3 countdown creatures, or even higher. cd:4 will bring you to state machine and clock library, and cd:5+ will get you to the units that don't naturally countdown. You can also use cd:2- for building faster decks.
this one is less known about, it's not on the dropdown menu in the deck builder, and it's the only one that isn't on the tip on the login screen that lists them. c: filters cost, referring to resource cost to play the unit/spell/enchant, so you could use c:6+ to find the more expensive scrolls, or c:3- for a cheap cost deck, or just put c:1 for 1 cost scrolls to add to your deck. And until wild is fully introduced, multi resource decks are generally better low cost, so you might want to use c:5- for it.
I assume l stands for level, but again, this one is less known about. Using l:1 will show you all your regular scrolls, where as you could use l:3 for all your tier 3 scrolls, or just l:2+ for all upgraded scrolls. this might be useful in trade, but not in standard deck building. it's always nice to know, though.
in my opinion this is the most useful filter by far, as it filters how many of each scroll you have, or a certain type. #:4+ will let you see what trade partners have spares of, if they don't sell things they have 3 or less of (which would be filtered with #:3-). #:5+ shows you what scrolls you can craft and still have a usable 3 of, and my favourite application of this filter, and by FAR the most useful, is #:2-. #:2- filters out all the scrolls you don't have 3 of, and as long as you have at least 1 of each, you will have a very easy time tracking down what you need, and how many of each you need, this filter is amazingly helpful in completing collections, and in the past someone even gave me 300g when I told them about this filter. If you combine #:4+ with r:r and optionally, s:5+ you can find the most valuable scrolls you own for a WTS list. You can even use the message 'WTB all my #:2- scrolls'. This filter has too many functions to list, so just know it exists, and use it everywhere, it, is, amazing.
I'm going to guess that fl stands for flavour, as it filters the flavour text, although I call it the lore personally. fl: is arguably the most useless, although it's cool and many people don't know about it. It filters text out of the lore/flavour text at the bottom of a card (the bit in italics), so you could find out everything Arran the machine priest has said, or Siani the Dyrran historian, or even some of the lore regarding the kinfolk. It isn't useful for trade or deck building, although you could amuse yourself with it.
Lastly, you can have as many filters filterin' at one time as you like, a few useful combinations are #:2- or #:4+ with r:r, or for finding creatures with the same stats to compare them (e.g. typing in c:3 ap:3 hp:3 to see all the 3 cost, 3 health and 3 attack creatures, and then just realise how good royal skirmisher/rot eater are, or type c:2 ap:2 hp:3 and realise that kinfolk ranger is worse than everything else in every way possible) You may also have multiples of the same filter, as long as they don't conflict. e.g. you can type in c:2+ c:5- to filter all scrolls between 2 and 5 cost, but you CAN'T type in c:2 c:5, as there's no scroll that costs both 2 and 5.
If I missed anything, let me know!
Combat, unit values, units attacking, and units being attacked is explained in this guide aimed at new players.