Different type of tiers:
Single tier:
In this type of tier all the three layers (Presentation layer, Application layer, Database layer) are installed in same system. It can be used in very small scale business requirements but it is not at all recommended to install the three layers in same system as it will reduce the performance, can't handle more no. of users, high hardware requirements, etc.,
Two tier:
In this type of tier two layers ( Application layer & Database layer) will be installed in one system and the Presentation layer will be installed in other system(usually end user's computer or laptop). This can be suitable for small and medium scale industries where the no. of end users are in optimal count.
Three tier:
All the three layers will be installed on different systems i.e. Database layer in separate system, Application layer in another system and Presentation layer in end user's computer or laptop. This kind of architecture will deliver excellent performance both in software and hardware vise. It can accommodate high number of users and will be more consistent. This type of architecture is recommended by SAP.
