Welcome to JUNOS. I'm sure, after a little while getting used to the differences, you'll love it :-)
There are some good books to start off with. I'd recommend Aviva Garrett's JUNOS Cookbook. It gives you a whole load of common scenarios and how to handle them in JUNOS. I'd also recommend Harry Reynolds and Doug Marschke's JUNOS Enterprise Routing.
The online documentation is very thorough and all available to anyone who wishes to take a look. You can get that at http://www.juniper.net/techpubs/software.
You might also want to take a look at the JUNOS Fast Track program at http://www.juniper.net/training/fasttrack/.
To answer your other question, "what's unit0 all about?"...
JUNOS describes interfaces either as physical interfaces, which take the form of <type>-<fpc>/<pic>/<port> (e.g. ge-1/0/3), and logical interfaces. The logical interfaces are configured using the unit <number> notation. Each unit is a new logical interface. Obviously, it depends upon the physical interace and encapsulation as to whether it makes sense to have more than one logical interface (e.g. a SONET interface with PPP encapsulation will only have unit 0, since PPP doesn't give a mechanism for logical separation, while a SONET interface running Frame Relay will have a logical interface per DLCI).
The complete logical interface is described as <type>-<fpc>/<pic>/<port>.<unit#>.
There is no implicit link between the logical unit number and the logical encapsulation identifier (VLAN tag, DLCI, etc) but it is not uncommon for people to use the same unit number as the logical encapsulation identifier.
thanks for your reply... Actually I would like to start with something like understanding the what kind of Physical interface and logical interfaces I will have in JUNOS supported routers... then what are the configuration modes are... what can i do at perticular mode...? and then I will start working on configuration level...
It definitely sounds like you would benefit greatly from some of the training offered as part of the fast track certification program. If you're already familiar with Cisco and IOS, it's the perfect way to go to get familiar with Juniper and JUNOS.
In reply to the specific questions regarding what physical interfaces and encapsulations we support, here's a brief (and not necessarily exhaustive) list...
Juniper routers come in a variety of sizes from the J2320 right up to the T1600. Obviously, not all interface types are available on all routers.
Ethernet: 10/100/1000 Copper, FE only Copper, FE Fibre, GE Fibre (SFP), 10GE Fibre (XFP or XENPAK)
SONET/SDH: OC3/STM1, Ch-STM1 to E1, OC12/STM4, Ch-OC12 to DS3, OC48/STM16, OC192/STM64, OC768/STM256
ATM: OC3/STM1, OC12/STM4, OC48/STM16
Others: E1, T1, E3, DS3, xDSL
There are also services PICs that don't have any physical ports on the outside but provide service type interfaces (tunnels, multilink services, encap/decap services, etc)
On the encapsulations, we support various VLAN oriented encaps on Ethernet. On SONET/SDH and En/Tn interfaces we support PPP, cisco-hdlc and frame-relay. On ATM, we support AAL5mux, AAL5snap, erm... running out here :-)
For a more complete review of each interface type, take a look at the online documentation for each interface type under the Network Interfaces section. For more basic review of how to get started with a JUNOS based box, System Basics is a good place to start.