go to the AdminConsole click on Domains->servers-monitor-
>Health

Weblogic server is based on the concept of admin and 
managed servers and node manager to monitor the servers. In 
websphere the terminology is bit different that they have 
the concept of cell and nodes and Development manager. you 
have to federated a node to a cell and the dmanager is 
analogous to the admin server in weblogic.

1)In WL 8.x we have connection pools and data sources, 
where as in WL 9&10 we have only data sources and inside 
data sources we are having connection pools.
2)In WL 8.x we ahve execute queue's, where as in WL 9&10 we 
are having work managers.
3)In WL 8.x we dont have LOCK & EDIT, Where as in WL 9&10 
we have LOCK & EDIT.
4)In WL 8.x licence.beafile is present to update the 
licence,Where as in WL 9&10 there is no licence.bea file.

All the above is correct except for #4.  You do need the
license.bea for WL 9&10.  The difference is that there is a
generic license.bea file for each version that you can
download from Oracle.  No need to request a custom
license.bea file.  In WLS 10 MP2 and above, the downloaded
software comes with a fully blown license so no need to even
download a new one from Oracle.

I am adding some more points:

5) In Weblogic 8.x, all the jms details will be present in 
the same config.xml whereas 9.x/10.x it will be partly 
maintained in config.xml and a separate xml files in 
domain/config/jms folders for each jms module.
6) In 8.x we don't have concept of JMS modules and 
subdeployments whereas its in 10.x.
7) In 8.x, we have queues,topics,distributed destination 
etc., separately whereas in 10.x they are clubbed inside a 
JMS module.
8) The directory structure inside a domain differs alot 
between 8.x and 9.x/10.x like the config.xml location,cache 
and staging folder paths,managed or admin server paths etc.
9)The wlst is made more easy and comfortable from 9.x 
onwards when compared to 8.1 sp6.

A WebLogic Server cluster consists of multiple WebLogic 
Server server instances running simultaneously and working 
together to provide increased scalability and reliability. 
A cluster appears to clients to be a single WebLogic Server 
instance. The server instances that constitute a cluster 
can run on the same machine, or be located on different 
machines. You can increase a cluster’s capacity by adding 
additional server instances to the cluster on an existing 
machine, or you can add machines to the cluster to host the 
incremental server instances.

1)A WLS clustr consists a group of server instances that 
work together and look like a single instance to the 
client.Cluster provides increased scalability, high 
availability, load balancing, reliability etc.Through 
multicaste adresses all the WLserver will communicate 
through Heart Beat.For ever few sec the WLS instances will 
release thir heart beats and if any of the server instances 
does not release the heart beat then that server in 
unavailable.And the stub also refreshes its list very 
periodicaly so that it can know the available server 
instances.
2)Yes, the other server will know through Heart Beats.

1. cluster is group of WLS instances which are capable of 
sharing loads (through load balancing) and providing high 
availability for all deployments. 

Weblogic instance uses the multicast address to 
commmunicate with each other (to communicate their status 
to other members of the cluster).

2. Yes, Server instances are aware of the accessibility of 
the other members of the cluster (as each server instance 
sends heartbeats on <multicast address>:<multicast port>) 
and hence are failover capable.

http://weblogicserver.blogspot.com/2007/11/administration-console.html

Hope this helps

it presents in client side environment

clients that connect to a WLS instances and look like a 
single clustered object obtain a replica-aware-stub of the 
object. The stub contains the list of all the available 
server instancesof the object.It also has a load balancing 
logic to distribute the load across the multiple hosts.

Domain structure includes the folders and files that are
created and appear when creating a domain.
Ex:- In WLS 8.1 domain structure is diferent like, it have
"applications" folder for auto-deployment.
Where as in WLS 9.0 and later versions, we have "autodeploy"
folder.

Servers instances and clusters includes in "Domain".Domain
is collection all these server instances and clusters
nothing but Domain.

Domain may be defined as scope of Admin server.
What all you have in your domain can managed by your Admin 
server.

Objects out of domain are not in scope on Admin server.

Yes, Cluster and servers are defined in domains and hence 
are managable from the Admin server.

Domain is the logical representation which consistis of one 
admin server per domain and the managed servers.
We can configure the clusters and server instances 
depending on the requirement.The imp file config.xml is 
present in admin server.And all the operations are managed 
from the admin server.

Troubleshooting of weblogic server:
fails to start weblogic instances
maintenance of Admin server as well all managed 
servers..etc...

Troubleshooting in weblogic is find out the cause(Server 
fail, Server shutdown, SSL issue, Thread stuck, etc…) of 
weblogic server failure if it occur by servers logs,

You can also troubleshoot the outofmemory(OOM) or server 
hanging issues by taking a thread dump of the server. in 
unix use kill -3 pid command. in windows i am not very 
sure. this gives a snapshot of the threads running in the 
server at the time of issue.

A domain is the basic administration unit for WebLogic 
Server instances.A domain consists of one or more WebLogic 
Server instances (and their associated resources) that you 
manage with a single Administration Server.We deploy the 
application in domain.Each domain's configuration is stored 
in a separate configuration file called config.xml, which 
is stored on the Administration Server along with other 
files such as logs and security files. When you use the 
Administration Server to perform a configuration task, the 
changes you make apply only to the domain managed by that 
Administration Server.

Almost all of the above information is correct as provided
in the 'E-Docs' of BEA. But in 'Weblogic 8.1' I proved that
more than Admin Server can exist for a domain and further a
domain can contain multiple Sub Domains. I have a doc.
explaining the procedure to do it.........

Do we deploy Application in Domain ? I guess we deploy it 
in Managed instances