[SalesForce] How to get Standard and Custom Object List in Apex class

From within an org itself, you can view it either via the Object definition or Schema Builder. For standard objects, you'll see them under "Customize->Object Name" in the setup menu (or just type the object name into the quick filter). Or in Schema Builder, you can just filter down to the objects want.

From within Apex, you can use the system level Schema object to do describes. More info here

From the API, you can use the meta data API to get information about customizations, or you can see them via the Enterprise version of the WSDL.

The Id field is a special case in that it exists on all objects. Documentation about the Id field can be found here.

Though not 100% an answer, but based on various posts and comments (like one from @Andrew), below is solution which will give you list of all standard and custom objects ignoring system objects:

public List < SelectOption > getStandardCustomIgnoreSytemObjects() {
    List < SelectOption > options = new List < SelectOption > ();
    for (Schema.SObjectType item1: Schema.getGlobalDescribe().values()) {
        String name = item1.getDescribe().getName();
        // Exclude all the unwanted Sobjects e.g. CustomSettings, History, Share, Feed, ApexClass, Pages etc..
        if (!item1.getDescribe().isCustomSetting() && item1.getDescribe().getRecordTypeInfos().size() > 0 && item1.getDescribe().isCreateable() &&
            !name.containsignorecase('history') && !name.containsignorecase('tag') && !name.containsignorecase('share') && !name.containsignorecase('feed')) {
            options.add(new SelectOption(item1.getDescribe().getName(), item1.getDescribe().getLabel()));
        }
    }
    options.add(new SelectOption('Asset', 'Asset')); // Asset doesn't come-up, so explicitly add same.
    options.sort();
}