There are two kinds of subtitle formats which the vobsub package can deal with: 
1. vobsub ripped: idx/sub[/rar]
2. textual: srt, sub, psb, smi, ssa, ass 

# of subtype you can use in
- VobSub: 1
- Textsub: 2
- Subresync: 1, 2
- DVobSub: 1, 2

Textual formats:

Notes:
- Watch out, the smallest syntax error and your file will be rejected!
- If a line is too long then it will be word-wrapped using {\q0}.
- [...] means that whatever is there it will be skipped (except subviewer's style modifers).

Styles/tags:
- microdvd: input
- sami: input, partial: <br>,<i>,</i>,<b>,</b>,<font color=...>
- ssa/ass: input/output
(if you want to see style support for other formats then send me their specification)

About SSA:

- There will be no animation effects in prebuffered mode of dvobsub.

About textual formats:

- Can't remember which format belongs to which ripper, so I just dump here a few samples from each.
- No support for multi-language SMI.
- There is an option to override the predefined "Default" style when opening srt, sub, ... using an additional file placed next to the subtitles (like somesub.srt and somesub.srt.style). It must use the format of ssa or ass and should only contain the style definition part without any dialogue lines. To have some hint how this is done, try to modify the default parameters of some .srt in subresync and re-save it.

------------------------------------

1
00:01:23,456 --> 00:12:34,567
Line 1!

2
01:01:23,456 --> 01:12:34,567
Line 2!
Line 3!

3
01:01:23,456 --> 01:12:34,567
Line 4!
[... empty line ...]
Line 5!

------------------------------------

{DEFAULT}{}{controlcodes} (optional)
{12345}{23456}Line 1
{56789}{}Line 2
{34567}{45678}Line 3|Line 4

All microdvd control codes are supported.

------------------------------------

{01:02:03}{02:03:04}Line 1
{03:04:05}{06:07:08}Line 2|Line 3

------------------------------------

[...] These info field will be skipped at the beginning

00:01:23.45,00:12:34.56
Line 1

01:01:23.45,01:12:34.56
Line 1[br]Line 2

Supported style modifiers:
[COLF]&Hbbggrr
[STYLE]no,bd,it,st,ud
[SIZE]height
[FONT]fontname

------------------------------------

0:01:23:Line1
0:02:34:Line2

------------------------------------

<SAMI>
[...]
<SYNC Start=24367><[...]>Line1
<SYNC Start=37682><[...]>Line2
[...]

Supoorted html tags:
<br>
<b></b>
<i></i>
<font color=...></font>

------------------------------------