There are 2 process do it with customization.

1. Using Formulas

Let teh field in which you want the concatenated value to be stored as Formula Field & just put a formula to concatenate say i want to concatenate Name and Number of Account into Billing city formula field then I would write the formula as  "Name  &  AccountNumber".

This will be a read only field & you cannot edit it.

2. Using Workflows


1. Go to workflows. Click New Rule. Select the Object say Account.
2. In Step 2, Give the Rule a proper name.Put criteia as Account Name is not equal to null which means for all accounts this workflow will run.Keep the evaluation criteria as Created and Edited.Click Next.
3. Add a workflow action, New Field Update.Give it a name and select the field in which you want the concatenation to be updated.
4. Select Use Formula to set the new value.
5. In the Formula editior, select the 2 fields which you want to concatenate in a single field. Say I want to concatenate the Account Name & Account Number in the Billing city. I will write in the formula "Name  &  AccountNumber" & click Save.
6. Done & Activate.
7. Once you Save a record the Account Name and Number will be concatenated and put in the Billing City.


Important :

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