DescribeSObjectResult: Check whether the object is Custom or Standard

List<String> allCustomObjsList = new List<string>();
List<String> allStandardObjsList = new List<string>();

for(Schema.SobjectType obj : Schema.getGlobalDescribe().values()) {
    Schema.DescribeSobjectResult objResult = obj.getDescribe();
    if(objResult.isCustom()) {
        String strObjName =objResult.getname();
        allCustomObjsList.add(strObjName);
    }
    else {
        String strObjName =objResult.getname();
        allStandardObjsList.add(strObjName);
    }
}

System.debug('Custom Objects List: ' +allCustomObjsList);
System.debug('Standard Objects List: ' +allStandardObjsList);

If you need to check for a specific object, execute the below code:

Schema.DescribeSObjectResult describeAccount = Schema.sObjectType.Account;
System.debug(describeAccount.isCustom());

Similarly you can use other methods for describing sObjects, the list of which can be found below:
https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_methods_system_sobject_describe.htm

Dynamic Apex

Describe information provides metadata information about sObject and field properties. For example, the describe information for an sObject includes whether that type of sObject supports operations like create or undelete, the sObject's name and label, the sObject's fields and child objects, and so on. The describe information for a field includes whether the field has a default value, whether it is a calculated field, the type of the field, and so on.

You can describe sObjects either by using tokens or the describeSObjects Schema method.

Apex Describe Information
• Token
   • sObject
      • sObject Token
   • Field
      • Field Token

• Describe Result
   • sObject
      • sObject Describe Result
   • Field
      • Field Describe Result

Using sObject Tokens

Schema.SObjectType is the data type for an sObject token. To access the token for an sObject, use one of the following methods:

1. Access the sObjectType member variable on an sObject type, such as Account.

Schema.sObjectType t = Account.sObjectType;

2. Call the getSObjectType method on an sObject describe result, an sObject variable, a list, or a map.

Account a = new Account();
Schema.sObjectType t = a.getSObjectType();

Details on getSObjectType(): https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_methods_system_sobject.htm#apex_System_SObject_getSObjectType

Using field Tokens

Schema.SObjectField is the data type for a field token. To access the token for a field, use one of the following methods:

1. Access the static member variable name of an sObject static type, for example, Account.Description.

Schema.SObjectField fieldToken = Account.Description;

2. Call the getSObjectField method on a field describe result.

// Get the describe result for the Name field on the Account object
Schema.DescribeFieldResult dfr = Schema.sObjectType.Account.fields.Name;

// Verify that the field token is the token for the Name field on an Account object
System.assert(dfr.getSObjectField() == Account.Name);

// Get the describe result from the token
dfr = dfr.getSObjectField().getDescribe();

Using sObject Describe Results

Schema.DescribeSObjectResult is the data type for an sObject describe result. To access the describe result for an sObject, use one of the following methods:

1. Call the getDescribe method on an sObject token.

Schema.DescribeSObjectResult dsr = Account.sObjectType.getDescribe();

2. Use the Schema sObjectType static variable with the name of the sObject.

Schema.DescribeSObjectResult dsr = Schema.SObjectType.Account;

Using Field Describe Results

Schema.DescribeFieldResult is the data type for a field describe result. To access the describe result for a field, use one of the following methods:

1. Call the getDescribe method on a field token.

Schema.DescribeFieldResult dfr = Account.Description.getDescribe();

2. Access the fields member variable of an sObject token with a field member variable

Schema.DescribeFieldResult dfr = Schema.SObjectType.Account.fields.Name;

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Accessing All Field Describe Results for an sObject

Map representing the relationship between all the field names (keys) and the field tokens (values) for an sObject.

Map<String, Schema.SObjectField> fieldMap = Schema.SObjectType.Account.fields.getMap();

Accessing All sObjects

Map representing the relationship between all sObject names (keys) to sObject tokens (values).

Map<String, Schema.SObjectType> gd = Schema.getGlobalDescribe();

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The following algorithm shows how you can work with describe information in Apex:

1. Generate a list or map of tokens for the sObjects in your organization.
2. Determine the sObject you need to access.
3. Generate the describe result for the sObject.
4. If necessary, generate a map of field tokens for the sObject.
5. Generate the describe result for the field the code needs to access.

Map<String, Schema.SObjectType> gd = Schema.getGlobalDescribe();
Schema.SObjectType t = gd.get('Broker__c');
Schema.DescribeSObjectResult dsr = t.getDescribe();
Map<String, Schema.SObjectField> fieldMap = dsr.fields.getMap();
Schema.DescribeFieldResult dfr = fieldMap.get('Name').getDescribe();

References:

1. DescribeSObjectResult Class
2. DescribeFieldResult Class
3. http://www.swdcworld.com/2017/06/salesforce-dynamic-apex-useful-code.html