The Glagolitic alphabet was almost entirely replaced during the Middle Ages by its descendant, the Cyrillic alphabet. Nowadays, it's only used for Old Church Slavonic in the service-books of Dalmatian Roman Catholics of the Slavonic rite[?].
While based mainly on mediaeval Greek, there are some letters of uncertain or unknown origin. Even the tradition that the alphabet was designed by the Greek monks Saint Cyrcil and Saint Methodius is not universally accepted. A less common belief is that the Glagolitic was created by St. Jerome, hence the name Hieronymian.
The alphabet has two variants: round and square. The round variant is dominated by circles and smooth curves, and the square variant features a lot of right angles, and sometimes trapezoids. See an image of both variants (incomplete) (http://kodeks.uni-bamberg.de/AKSL/Schrift/GlagolVergleichAlphabet.htm).
The name comes from the Slavonic glagol, meaning word (which was also the name for the letter "G"). Since glagolati also means to speak, the Glagolitsa are poetically referred to as "the marks that speak".
The following table lists each letter in order, giving a picture (round variant), its name, its approximate sound in SAMPA, the Greek letter that it was used to transliterate (if applicable), and the modern Cyrillic letter that it directly gave rise to (if any).
| Picture | Name | Sound | Relation to Greek | Relation to modern Cyrillic |
|---|---|---|---|---|
 | Az | /a/ | Alpha | A |
 | Buki | /b/ | Be | |
 | Vedi | /v/ | Beta | Ve |
 | Glagol | /g/ | Gamma | Ghe |
 | Dobro | /d/ | Delta | De |
 | Jest | /E/ | Epsilon | E |
 | Zhivete | /Z/ | Zhe | |
 | Dzelo | /dz/ | Macedonian Dze | |
 | Zemlja | /z/ | Zeta | Ze |
 | Izhe | /i/ | Eta | I |
 | I | /i/ | Iota | Ukrainian I |
 | Dzherv | /dZ/ | Serbian Šerv | |
 | Kako | /k/ | Kappa | Ka |
 | Ljudi | /l/ | Lambda | El |
 | Mislete | /m/ | Mu | Em |
 | Nash | /n/ | Nu | En |
 | On | /O/ | Omicron | O |
 | Pokoj | /p/ | Pi | Pe |
 | Rtsi | /r/ | Rho | Er |
 | Slovo | /s/ | Sigma | Es |
 | Tverdo | /t/ | Tau | Te |
 | Uk | /u/ | Omicron Upsilon | U |
 | Fert | /f/ | Phi | Ef |
 | Kher | /x/ | Chi | Ha |
 | Oht | /o/ | Omega | Only used to transcribe Greek |
 | Tsi | /ts/ | Tse | |
 | Cherv | /tS/ | Che | |
 | Sha | /S/ | Sha | |
 | Shta | /St/ | Shcha | |
 | Jer | /w/, /@/ | The hard sign | |
 | Jery | /1/ | Jery | |
 | Jerj | /j/ | The soft sign | |
 | Jat | /j{/ | Removed from Bulgarian in 1945 | |
 | ? | /jO/ | Obsolete | |
 | Ju | /ju/ | Ju | |
 | Jus Malij | /E~/ | Obsolete | |
 | Jus Malij Jotirovannij | /jE~/ | Obsolete | |
 | Jus Bolshoj | /O~/ | Obsolete | |
 | Jus Bolshoj Jotirovannij | /jO~/ | Obsolete | |
 | Thita | /f/ | Theta | Only used to transcribe Greek |
 | Izhitsa | /v/, /i/ | Upsilon | Removed from Russian in 1917 |
I'm not sure of the name of the letter between Jat and Ju.
Note that Jery is simply a ligature of Jer and I. The order of Izhe and I varies from source to source, as does the order of the various forms of Jus.