Encoding is the process of writing data to an NFC tag, in a similar way that you might write data to a USB stick. For example, you may encode a web address (URL), some text or a command.
On a small scale, NFC tags can be easily encoded with an NFC enabled mobile phone. For large scale encoding, RapidNFC offer a fast and reliable NFC tag encoding service capable of encoding over 200,000 tags per day.
To ensure that an NFC tag encoded with one phone can be read by another, encoding follows a set of guidelines set up by the global NFC body - the NFC Forum. RapidNFC is a member of the NFC Forum and all tags encoded by RapidNFC adhere to these guidelines.
The answer to that usually depends how many other people you want to be able to read your tags. All NFC enabled phones currently on the market can read a web address or text. The vast majority will understand a telephone number or short SMS message, most will understand an email. If it's only yourself reading a tag, then 'Task Launcher' data (such as commands to turn on/off WiFi) can be easily encoded as well.
More complex encoding such as vCards is easily possible using common apps, but compatibility issues will mean that you can't always be sure how other people will see your data.
Yes. Encoding and Locking NFC tags are essentially two different actions so you can encode and re-encode tags as many times as you like. Once you are happy with the content, you can lock the tags to prevent alteration. This can be done using the same mobile apps.
Once a tag has been properly locked, it cannot be changed or 'hacked' by anyone.