Kana are the most fundamental characters in the Japanese language, analagous to the English alphabet. Each kana represents a syllable like "ka" or "ni". There are 46 different Kana, but each one has two different ways it can be written (if this seems confusing, consider that English has lowercase and capital letters). These are called Hiragana and Katakana.
HiraganaThe more common variant, and is the default for most situations. It generally looks round and loopy.
KatakanaMostly used for foreign loanwords. It generally looks sharp and angular.
In the Reference tab, there is a kana chart using the same configuration as can be found on learning materials in Japan. Each column represents a set of kana sharing a starting consonant and each row represents a set of kana sharing the same ending vowel (k + i = ki).
You may notice that the first column (starting from the right side) has a blank consonant. Those are the kana "a, i, u, e, o" that have no consonants. The last column labeled "nn" technically has no vowel as it represents the sound "n". It"s labeled as such to disambiguate it from the other "n" column.
Kana with "k, s, t, h" consonants can have "dakuten" which are two small marks in the top-right (は > ば) that change the sound from unvoiced (ha) to voiced (ba).
Kana with an "h" consonant can alternatively have "handakuten", which is a small circle in the top-right (は > ぱ). This changes it to a "p" sound (pa).