Quote Originally Posted by starrysky
The "твёрдый знак" Ъ is indeed more or less unimportant, it is mostly used in orthography to show that the preceding consonant is hard ("твёрдый" = hard) and there are some other functions but I don't remember them.
The difference is in sound of suсceeding vowel - подъехать - [pɐdjɛxətʲ], подехать (no such word) - [pɐdʲɛxətʲ]
In the second case 'd' is soft and in the first there is 'j' sound before ɛ