あ
a
あめ
ame
rain
Hiragana and katakana are both Japanese syllabic scripts (kana), each with ~46 base characters representing the same set of sounds. The difference is their use:
Hiragana (ひらがな) — rounded, cursive shapes
Katakana (カタカナ) — angular, sharp shapes
Both are phonetic and encode the same sounds — they're parallel systems.
What are those accent marks on some of the characters? Those "accent marks" are diacritic marks that change the sound of kana, mainly called dakuten (゛) and handakuten (゜) in Japanese writing. Click the 濁点 tab below to see it.
The dakuten — two small strokes at the top-right of a character — voices the consonant: k→g, s→z, t→d, h→b. The handakuten — a small circle — only appears on H sounds (ha hi fu he ho), turning them into P sounds (pa pi pu pe po). Together they add roughly 25 more syllables and work identically in both hiragana and katakana.
* ぢ and づ (and their katakana ヂ/ヅ) are archaic forms that sound identical to じ (ji) and ず (zu) in modern Japanese. They appear only in a handful of words where the voiced consonant comes from compounding or reduplication (e.g. ちぢむ, つづく).