Divisions of Codes and Ciphers
There are two kinds of codes/ciphers: transpositional, where your characters are the same, just moved about (like an anagram with a precise way to revert the encrypted text to the message), and translational, where your characters are in the correct position, but it's not the same characters (like a masquerade party).
The earliest transpositional cipher one would normally think of is good old Scytale. Imagine you have a tube, and wrapped around said tube is a strip of cloth. You have to make two decision: - How many sides does the Scytale have? - How many times can you wrap the strip of cloth around the tube? You have that? Good. Now, you write your message on the strips on the tube. For me, my Scytale has six sides, and I can wrap my strip four times around. After you've done this, you can unwrap the strip of cloth. What you should see is a bunch of characters with no rhyme or reason to it. The receiver, on receiving the strip of cloth, would wrap it around a tube of similar dimensions, and they would be able to read the message. My encrypted text: WSEARLIHSYULLIAFSLNWOA
The earliest translational cipher one would normally think of is our friend Caesar. Shift forwards/backwards by a number less than 26 (the number of letters in the English alphabet) to get a completely new one. My encrypted text: JURA GURER'F N JVYY, GURER'F N JNL.
While these are good in their time, there's just one problem if you use it now: It can be bruteforced. What do I mean by bruteforce? Well, let's look at the case for Caesar.
Since Caesar only has 26 possible configurations (assuming you're using the normal, unchanged alphabet), you can simply write out all 26 configurations, and see which one looks the most intelligible. For "JURA GURER'F N JVYY, GURER'F N JNL", it'll end up something like this.
KVSB HVSFS'G O KWZZ, HVSFS'G O KOM.
LWTC IWTGT'H P LXAA, IWTGT'H P LPN.
MXUD JXUHU'I Q MYBB, JXUHU'I Q MQO.
NYVE KYVIV'J R NZCC, KYVIV'J R NRP.
OZWF LZWJW'K S OADD, LZWJW'K S OSQ.
PAXG MAXKX'L T PBEE, MAXKX'L T PTR.
QBYH NBYLY'M U QCFF, NBYLY'M U QUS.
RCZI OCZMZ'N V RDGG, OCZMZ'N V RVT.
SDAJ PDANA'O W SEHH, PDANA'O W SWU.
TEBK QEBOB'P X TFII, QEBOB'P X TXV.
UFCL RFCPC'Q Y UGJJ, RFCPC'Q Y UYW.
VGDM SGDQD'R Z VHKK, SGDQD'R Z VZX.
WHEN THERE'S A WILL, THERE'S A WAY
XIFO UIFSF'T B XJMM, UIFSF'T B XBZ.
YJGP VJGTG'U C YKNN, VJGTG'U C YCA.
ZKHQ WKHUH'V D ZLOO, WKHUH'V D ZDB.
ALIR XLIVI'W E AMPP, XLIVI'W E AEC.
BMJS YMJWJ'X F BNQQ, YMJWJ'X F BFD.
CNKT ZNKXK'Y G CORR, ZNKXK'Y G CGE.
DOLU AOLYL'Z H DPSS, AOLYL'Z H DHF.
EPMV BPMZM'A I EQTT, BPMZM'A I EIG.
FQNW CQNAN'B J FRUU, CQNAN'B J FJH.
GROX DROBO'C K GSVV, DROBO'C K GKI.
HSPY ESPCP'D L HTWW, ESPCP'D L HLJ.
ITQZ FTQDQ'E M IUXX, FTQDQ'E M IMK.
JURA GURER'F N JVYY, GURER'F N JNL.
As you can see, the 13th one in the list is the one that is the most legible. If you don't know a key, you can bruteforce it. If there's not a lot of possibilities, then you get this situation, which both Scytale and Caesar has. Scytale fares a bit better, but it is still bruteforceable.
But there is a way to make the Caesar a bit stronger: key it. There are 26 letters of the alphabet, so there are 26! (26 factorial, basically the product of every number from 1 to 26) alphabets to choose from. The normal progression of the alphabet is but one of the ways to choose.
The problem with that, however, is that it's still very, very crackable. Anyone who has played Cryptogram before knows how easy it is to crack it without a key. Even if you have to use bruteforce to check, it's still rather solvable.
So we have to ramp it up somehow, but how? Enter: bigrams, trigrams, and polyalphabetic ciphers