Internet Flat Rate is the access to the Internet all the hour and days of the year (lineal rate) and for all the customers of the telco operator (universal) at a fixed and cheap tariff.
Flat rate is habitual in all the broadband access to the Internet. To be universal, flat rate must be offered to the dial-up access users, not only to brodband Internauts.
A "charge tariff" is a class of lineal rate, different from the flate rate, where the user is charged by the uploads and downloads (data transfers). The GPRS access to the Internet in some countries of Europe has not fat rates pricing, following the obsolete metered mentality. Because of this, the users prefer utilize the fixed lines ( with narrow or broadband access) to connect to the Net.
Wavy rate is not a lineal rate, because the Internet surfer pays the monthly fix price to use the connection only during a certain range of hours of the day (i.e. only in the morning or, more habitually, only at night).