You mixing in your questions alot of different features of Netsuite. Im going to try and explain them one by one. Im going to assume you know how to create script records and are somewhat familiar with NetSuite
Restlets and Suitelets
These are both hosted in NetSuite by creating script records. The main difference between the two is:
Restlets: Can be used with get, post, put, delete requests, this makes it ideal if you are building and API which you can use in another application.
Suitelets: Can be used with get, post. Can be used along with Netsuites server widgets to make native looking Netsuite HTML pages. Can also technically be used for as an API
SuiteScript
Sounds scary but this is NetSuites scripting language built on top of Javascript (ES5 for now). If you know JS you shouldnt have an issue learning SuiteScript. The Restlets and Suitelets make use of SuiteScript. SuiteScript is also used for example in what netsuite calls "User Event Scripts". You can have custom SuiteScript code execute on events like "beforeLoad", "afterSubmit", "beforeSubmit"
Workflows
So we mention User Event Scripts. Well these are scripts that only a programmer can create. Workflows allows an average user to execute actions using diagrams kind of like a visual user event script.
SuiteTalk
Unlike Restlets and Suitelets, SuiteTalk is and independent app built outside of NS. I believe they have support for C#, Java and maybe PHP. These are built using a WSDL provided by NS.
Can we Implement multi threading using schedule script for 1000 search requests(100 search requests on each thread).if yes,then how can we implement it?
I generally just execute the search again if theres more there is 1000 results and keep merging the lists untill its done.