SDLC is SOFTWARE DEVELOPMENT LIFE CYCLE.There are different 
phases in SDLC.They are:
1.PROJECT PLANNING
2.SYSTEM ANALYSIS,REQUIREMENT DEFINITION (BRD/FRD)
3.SYSTEM DESIGN(ARCHITECTURE/FLOW CHARTS)
4.CODING/PROGRAMMING/DEVELOPMENT
5.TESTING(SANITY,INTEGRESSION,REGRESSION,SYSTEM,PERFORMANCE 
TESTING)
6.UAT(USER ACCEPTANCE TESTING)
7.PRODUCTION/GO-LIVE
8.MAINTENANCE/SUPPORT

STLC is SOFTWARE TESTING LIFE CYCLE.There are different 
phases in STLC.They are:
1.PREPARATION OF TEST STRATEGY.
2.PREPARATION OF TEST PLAN
3.PREPARATION OF TEST CASES
4.TEST CASE REVIEW
5.EXECUTION OF TEST CASES
6.RESULT ANALYSIS
7.BUG TRACKING
8.TEST RESULTS REPORTING(PASS, FAILED TEST CASES/BUG 
REPORTING)
9.REGRESSION TESTING

I think what prasanna posted is not a perfect answer. what 
she had provided is good but the difference is 

SDLC is the process prepared and followed by the 
development team to work on the project in different phases 
which also includes STLC 

STLC is a process followed by the testing team to test the 
developoed project 
STLC is prepared by the testing team

I think this is a better answer.

Hi,pavan kumar.
Your answer is absolutely correct.
I have a doubt. 

From which stage, the testing process will start in SDLC.

 I mean, In SDLC stage, from which stage the tester cast 
his role?

STLC is a part of SDLC.

Testing starts from the requirementphase itself.( Here we 
will participate in requirements review)

But the execution of our tests starts from the testing 
phase itself.

Dear pawan,

testing start from strategic phase.then prepare a plan/case/
and go through 1.PREPARATION OF TEST STRATEGY.

2.PREPARATION OF TEST PLAN
3.PREPARATION OF TEST CASES
4.TEST CASE REVIEW
5.EXECUTION OF TEST CASES
6.RESULT ANALYSIS
7.BUG TRACKING
8.TEST RESULTS REPORTING(PASS, FAILED TEST CASES/BUG 
REPORTING)
9.REGRESSION TESTING

SDLC and STLC both are diff boss.