Usually you'll have a sediment, carbon, membrane, and DI stage. The sediment filter takes care of physical "large" impurities. The carbon removes unwanted chemicals like chorine. The RO membrane is what filters out small impurities and is where you'll see the bulk of the TDS(Total Dissolved Solids) removed from the water. The DI stage traps charged particles and is what gets you that last little way to 0 TDS. If you have good water coming in, say your tap is less than 100 TDS, then your DI resin will usually last a while. If you have very high TDS like 300-400 or more, than you'll exhaust your resin at a comparably faster rate.
Hope that basic explanation works. As for a good filter? Air Water Ice is a sponsor so you might want to take a look at their setups.
Edit: Usually you'll have a sediment, carbon, membrane, and DI stage. The sediment filter takes care of physical "large" impurities. The carbon removes unwanted chemicals like chorine. The RO membrane is what filters out small impurities and is where you'll see the bulk of the TDS(Total Dissolved Solids) removed from the water. The DI stage traps charged particles and is what gets you that last little way to 0 TDS. If you have good water coming in, say your tap is less than 100 TDS, then your DI resin will usually last a while. If you have very high TDS like 300-400 or more, than you'll exhaust your resin at a comparably faster rate.
Hope that basic explanation works. As for a good filter? Air Water Ice is a sponsor so you might want to take a look at their setups.