Form means the sound, shape and appearance of something, e.g. two forms of the word please, are pleases and pleased. The form of the sentence, e.g. 'He pleased himself.' can be explained by referring to two kinds of structure: that of its individual words (i.e. their morphology) and the way its words relate to each other (i.e. their syntax). The study of both of these aspects of sentences is called grammar. The study of the form of a text is called discourse analysis.
Formality is an important aspect of register. Informal language is friendly, familiar, and casual. Strict accuracy and correctness are not important in informal language. Formal language is authoritative, and more impersonal.
Speech tends to be informal, Writing tends to be formal.