A ROBLOX script builder or SB is a game where you can run code in-game. This is a place for basic to very advanced scripters. Script builders are a common hangout for scripters. Some SB's allow you to use your own scripts you made. If you don't and people see that you used a leak script or FM for short, they will call you a FMer and will probably kick/ban you. Most SB's that require you to use your own scripts have an age limit to join, usually 1 year or older to join. The most popular SB's are usually the ones that have FMers like Voidacity's Script Builder. Probably because you can use leaked scripts or FM's on it without getting banned or kicked.
Script builders are different now than they were in the past. Commands have changed and are different from previous script builders. For example, if you wanted to run a script you would use a command like so:
Also, there is a change in commands for local scripts. They run like so:
You can also run locals similarly:
please take note that the commands above are only for voidacity script builder
Most common way to store code for use in a script builder is using Pastebin  .
New script builders have what's called a sandbox. A sandbox is an environment made to execute unsigned code, meaning user generated code. If you try to kick a player, the sandbox blocks the kick. A sandbox is a very important and a part of modern-day script builders.
A script builder sandbox isn't perfect, as there are many people creating what are called bypasses. Bypasses allow the user to execute code outside the sandbox, allowing things such as kicking a player. Bypasses are usually made by advanced scripters and are usually kept a secret.
A breakout is a piece of code that allows access to the main environment, which means a user can access code that is ran in the game by intercepting commands such as "h/" or "hl/", allowing the user to see their code when the code is ran.
Although, sandboxes may have their weaknesses it is not very complex to fix those vulnerabilities, if the creator of the sandbox knows what he's doing.
Update: Many script builders are being required to filter interfaces made by other people. This means having a script that looks for messages, hints, and anything in a graphical user interface to ensure that it is filtered so that the game follows the ROBLOX community guidelines
What Happened to the Old Script Builders?
The old script builders used loadstring, which is disabled by default. Client-sided loadstring is not allowed, but server-sided loadstring is. That is why the old script builders will never work again. Anaminus's script builder was open source, until its closure in 2014, the year that ROBLOX patched loadstring. Ever since there have been countless users creating new script builders using Anaminus' code, which doesn't work, even though they claim client-sided loadstring works.
The FE transition
For a while, ROBLOX has been planning on transitioning games from the classic client/server model to a new model called "Filtering Enabled". This new model limits client sided code by not allowing changes to replicate to the server. This deems LocalScripts limited to UI/Input related functionalities. Since a lot of scripts developed for Script Builder specificially depended on the classic replication, this change rendered them useless. However, tusKOr661 has been moving to have Void SB be able to handle this change, and incorporate filtering enabled into client sided scripts. While this is still a work in progress, the community itself has stepped in to make some scripts FE compatible. However, many of the script builders could not handle this, rendering them useless. Roblox as of recently, has forced filtering onto every game.
List of Working Script Builders
Some of these may be under review
Void's Script Builder 
Please add more script builders that are usable in a filtering enabled environment.