SDLC MEANS SOFTWARE DEVELOPMENT LIFE CYCLE.
IN THIS WE HAVE FOUR MODELS
1.WATERFALL MODEL
2.PROTOTYPING MODEL
3.EVOLUTIONARY DEVELOPMENT MODEL
4.SPIRAL MODEL
5.SYNCHRONIZE-AND-STABILIZE MODEL

PROTOTYPE MODEL: THIS MODEL IS APPLICABLE FOR THE COMPANY 
WHEN THE USER CUSTOMER REQUIRMENTS ARE NOT CLEAR.WHEN THE 
PROTOTYPE IS DEMONSTATED TO THE USER,THE  USER GETS CLARITY 
OF THE FUNCTIONALITY OF THE SOFTWARE AND SUGGETS REQUIRED 
MODIFICATIONS.
 PROTOTYING IS GENERALLY REQUIRED TO OBTAIN USER INTERFACE 
REQUIRMENTS,BASED ON THE FEEDBACK, A MORE ACCURATE SRS 
DOCUMENT CAN BE PREPARED AND THE DEVELOPMENT WORK CAN 
START,USING THE WATERFALL MODEL.

SPIRAL MODEL: THIS MODEL IS APPLICABLE FOR THE COMPANIES 
WHEN THE USER REQUIRMENTS ARE INCOMPLETE.IN THE FUTURE,THEY 
MAY GET SOME REQUIRMENTS.THIS IS USEFUL FOR HIGH-RISK 
PROJECTS.
EACH LOOP IN SPIRAL REPRESENTS A DEVELOPMENT PHASE
1.PLANE THE NEST PHASE
2.OBJECTIVES ALTERNATIVES CONSTRAINTA
3.SRS
4.DESIGN
5.DETAILED DESIGN
6.EXECUTION
7.RISK ANALYSIS EVALUATION OF ALTERNATIVES
EACH LOOP IS SPLIT INTO FOUR SECTIONS,ONE FORM HAS TO BE 
FILLED CONTAINING THE FOLLOWING
A. TO DETERMINE THE OBJECTIVES,ALTERNATIVES,AND CONSTRAINTS
B. RISK ANALYSIS ABD EVALUATAION OF ALTERNATIVES
C.EXECUTION OF THAT PHASE OF DEVELOPMENT
D. PLANNING THE NEXT PHASE