Your second question is more complicated again.
';' is called a 'semicolon' in English, and many native-speakers can't use it correctly. The basic rule, which I follow, is if you're unsure, don't use it since it doesn't actually change the meaning at all. You can't use ',' to connect sentences unless followed by a conjunction, such as 'but' or 'and'. If you use one of these words, you must use a ','. A semicolon connects closely related sentences, but you shouldn't use a conjunction.
Many English speakers break this rule even in formal essays and this practice is becoming more accepted; this is especially common for younger generations who use ';' as a stronger ','. This is similar to another correct usage of the semicolon - listing items that have commas in them. For example 'I can buy a banana, an apple, and an orange; two bananas and an apple; or three bananas.' The semicolon clearly marks out the three options. In the same way, a semicolon can be used in a sentence with many commas to make the different clauses stand out, for example 'John, who had just arrived at the store, wanted to buy some fruit; but after looking at the rotting apples, bananas, and oranges that the store was selling, he changed his mind.' This is widely accepted as correct but only when writing long sentences. (I've only ever met two people that regard this usage as unacceptable. Some say it's debatable, but most find it correct.)
':' is called a colon, and is used to introduce a list or an example. It's also used when writing movie titles, such as 'Terminator 2: Judgement Day'.
コメント中に、ご本人が「；」を使ってくれているあたり、若い大学生の方です :) 。 Mitchellさん、どうもありがとうございました。