Many scholars believe that the Thai script is derived from the Khmer script, which is modeled after the Brahmic script from the Indic family. However, in appearance, Thai is closer to Thai Dam script, which may have the same Indian origins as the Khmer script (Sources needed here). The language and its script are closely related to the Lao language and script.

Most literate Lao are able to read and understand Thai, as more than half of the Thai vocabulary, grammar, intonation, vowels and so forth are common with the Lao language. We may say that Thai and Lao languages are cognate.

Much like the Burmese adopted the Mon script (which also has Indic origins), the Thais adopted and modified the Khmer and Mon scripts (and many others) to create their own writing system. While in Thai the pronunciation can largely be inferred from the script, the orthography is complex, with silent letters to preserve original spellings and many letters representing the same sound.

While the oldest known inscription in the Khmer language dates from 611 CE, inscriptions in Thai writing began to appear around 1292 CE [Sources needed].

Thai alphabet

Consonants

No.  Symbol   Name    

1      ก        ก ไก่         ko kai (chicken)    [  k ,  k ]

2      ข        ข ไข่         kho khai (egg)     [kh ,    k]

3      ฃ    ฃ ขวด     kho khuat    kh     k

4     ค     ค ควาย     kho khwai     kh     k

5     ฅ     ฅ คน     kho khon  [ kh     k]    

6     ฆ     ฆ ระฆัง     kho ra-khang (bell)     kh     k    

7     ง     ง งู     ngo ngu (snake)     ng     ng    

8     จ     จ จาน     cho chan (plate)     ch     t   

9     ฉ     ฉ ฉิ่ง     cho ching (cymbals)     ch     -    

10     ช     ช ช้าง     cho chang (elephant)     ch     t    

11     ซ     ซ โซ่     so so (chain)     s     t    

12     ฌ     ฌ เฌอ     cho choe (bush)     ch     -     I

13     ญ     ญ หญิง     yo ying (woman)     y     n    

14     ฎ     ฎ ชฎา     do cha-da (headdress)     d     t    

15     ฏ     ฏ ปฏัก     to pa-tak (goad, cattleprod spear)     t     t    

16     ฐ     ฐ ฐาน     tho san-than (base)     th     t    

17     ฑ     ฑ มณโฑ     tho nangmon-tho     th     t    

18     ฒ     ฒ ผู้เฒ่า     tho phu-thao (elder)     th     t    

19     ณ     ณ เณร     no nen (novice monk)     n     n    

20     ด     ด เด็ก     do dek (child)     d     t    

21     ต     ต เต่า     to tao (turtle)     t     t   

22     ถ     ถ ถุง     tho thung (sack)     th     t    

23     ท     ท ทหาร     tho thahan (soldier)     th     t   

24     ธ     ธ ธง     tho thong (flag)     th     t    

25     น     น หนู     no nu (mouse)     n     n    

26     บ     บ ใบไม้     bo baimai (leaf)     b     p    

27     ป     ป ปลา     po pla (fish)     p     p    

28     ผ     ผ ผึ้ง     pho phueng (bee)     ph     -    

29     ฝ     ฝ ฝา     fo fa (lid)     f     -    

30     พ     พ พาน     pho phan (tray)     ph     p    

31     ฟ     ฟ ฟัน     fo fan (teeth)     f     p    

32     ภ     ภ สำเภา     pho sam-phao (sailboat)     ph     p    

33     ม     ม ม้า     mo ma (horse)     m     m    

34     ย     ย ยักษ์     yo yak (giant)     y     y    

35     ร     ร เรือ     ro ruea (boat)     r     n    

36     ล     ล ลิง     lo ling (monkey)     l     n    

37     ว     ว แหวน     wo waen (ring)     w     w    

38     ศ     ศ ศาลา     so sala (pavilion)     s     t    

39     ษ     ษ ฤๅษี     so rue-si (hermit)     s     t    

40     ส     ส เสือ     so suea (tiger)     s     t    

41     ห     ห หีบ     ho hip (chest)     h    

42     ฬ     ฬ จุฬา     lo chu-la (kite)   

43     อ     อ อ่าง     o ang (basin)    

44     ฮ     ฮ นกฮูก     ho nok-huk (owl)     h    

Source: http://en.wiktionary.org/wiki/Appendix:Thai_alphabet

Thai has 44 consonants. They are phonograms, or sound signs. One should bear in mind that a phonogram is a grapheme (written character) which represents a phoneme (speech sound) or combination of phonemes, such as the letters of the Latin alphabet or the Japanese kana.

For example, "igh" is an English-language phonogram that represents the hard "I" sound in "high." Whereas the word phonics refers to the sounds, the word phonogram refers to the letter(s) that represent that sound.

Unlike Chinese characters which are ideograms or pictograms, Thai signs (symbols) represents sounds. Note that an ideograph is a graphic symbol that represents an idea or concept.

To learn Thai, you should visit the websites recommended below:

http://www.fsi-language-courses.org/Content.php?page=Thai