TDS is the measuring of the amount of salts in a solution. For alot of applications the amount of salt is indicitive of the levels of other stuff in a solution. TDS/PPM meters sold for gardening and aquariums figure the amount of salt in Parts Per Million by measuring the Electrical Conductivity of the solution under test. So a PPM/TDS meter is a EC meter that converts the EC value into PPM values. There are alot of descriptions online about why and how to measure PPM/TDS or EC so google for them if you want alot more details.
EC/TDS/PPM Meter On Limited Budget - [Link]