Sooner or later, you're going to get a huge spreadsheet with way too much data to scan visually.
How do you, ahem, filter out what you don't want to see?
One way is to use the AutoFilter.
AutoFilter Basics
Let's say you've got this spreadsheet of employees.
You'd like to just take a look at those from Montana, or those with a particular number of years of service. Something like that.
Click somewhere in the data, and choose Data > Filter > AutoFilter.
You see arrows by all the headings.
Click on one of the arrows, and choose to view all records containing one of the values, or all records containing the top 10, i.e. the ten most frequently occurring values in that column.
Here are the results for selecting one value for one column.
If you choose another value in another column, then you get rows that have the selected value for BOTH columns.

Here, I get rows for people who are in Montana, AND in Kalispell. Which works out fine since Kalispell is a city in Montana.
However, if I choose to view records for people from Montana, and from Portland (a city in Maine and in Oregon but NOT in Montana), I get nothing.
To go back to viewing all the values, select All from the list.
Then you get to view all the records again, once you've selected All for any columns you restricted.
When you're done and want to get rid of the little arrows, click somewhere in the data, and choose Data > Filter > AutoFilter again. There'll be a checkmark and when you select AutoFilter, it will go away.
There's the data the way it was before you started.
Issues With AutoFilter
Here's where things get a little twitchy. What if you try to turn off the AutoFilter and you have not selected a cell somewhere within the AutoFilter results?
You get this.
When you get this, click OK and click somewhere in the data.
Then choose Data > Filter > AutoFilter again. You won't see the checkmark, but that's OK.
Then choose Data > Filter > AutoFilter yet again. This time you'll see the checkmark.

And then the arrows will disappear and you're back to normal.

I also recommend liberal use of the Undo feature, Ctrl Z or click the Undo icon. You can undo at least 20 and possibly more depending on how your system is set up.
Can You Delete Rows When in the AutoFilter Without Deleting the Intervening Data?
I'm glad you asked. Yes, you can. Here's a demo. Look at the range from row 15, Dan Montbatten, to row 20, Beth Jerlin. They're both from Montana. In between you've got Jon, Marcus, and Kyle.
I'm going to view only people from Montana, which includes Dan and Beth but excludes the three rows between.
Now I'm going to delete Dan and Beth.
And they go away. However, Jon, Marcus, and Kyle are still there.
Thanks to Huw for the tips. For information on the sites, see:
http://www.openoffice.org/issues/show_bug.cgi?id=33851
http://wiki.services.openoffice.org/wiki/Calc/Drafts/Issue_33851
http://wiki.services.openoffice.org/wiki/Talk:Calc/Drafts/Issue_33851
Let's say you've got a big list of employees: name, address, etc. When you apply the filter and view, for instance, only people from Montana, some of the data isn’t shown. If you then copy, paste, delete, or perform other operations on the data, what happens to the data that isn’t shown? If you delete Artie Anderson from Montana and Cindy Chalmers from Montana, what happens to Betsy Bates from Nevada?
It all depends on the operation. Some, like delete, leave the unshown data alone. Some do affect the unshown data.
Operations that DO affect filtered out rows.
Cut and Paste
Move (dragging)
Fille (Edit > Fill or dragging)
Operations which do NOT affect filtered out rows:
Copy
Delete contents
Delete row
Format
Find & Replace in current selection
More About Operations that DO Affect Filtered-Out Rows
Cut and Paste, Versus Copy and Paste
With Cut and Paste, the non-shown data is cut and also pasted.
Click the following to see a bigger image.When you paste the data outside of the filter range, the whole set of data is pasted and unaffected anymore by the filter.
When you paste the data inside the filter range, the whole set of data is pasted, and all data is shown at first, even data that shouldn’t show for the current filter selections. However, if you re-apply the filter selections using the dropdown lists in the heading row, then the data is filtered correctly.
With Copy and Paste, the behavior is different. If you cut, you get the nonshown rows, but if you copy, you don’t.
Dragging Cells to Move Them
When you move rows (dragging) that include unshown rows, the behavior is the same as cut and paste. The unshown rows between shown rows are moved along with the shown rows.
Fill (Edit > Fill or Dragging the Cell Handle)
Here’s what happens with Fill. Here’s some sample data, and currently everyone is in the same department.
You now look at only people from Colorado.
You change the department for the first person from Colorado, and drag that department down through all the other people from Colorado.
Now, all the nonshown rows after the first row you changed are affected, but not the rows before that.
More About Operations that Don’t Affect Filtered-Out Rows
Copying and pasting, deleting, formatting, and Find and Replace don’t affect unshown data. The following section provides an example.
Deleting
Here’s a walkthrough of deleting rows while the filter is on. Rows that aren’t shown aren’t affected.
Look at the range from row 15, Dan Montbatten, to row 20, Beth Jerlin. Dan and Beth are both from Montana. In between are Jon, Marcus, and Kyle.
The next illustration shows an AutoFilter with only people from Montana, which includes Dan and Beth but excludes the three rows between.
Now delete Dan and Beth.
And they go away. However, when the state autofilter criterion is removed, Jon, Marcus, and Kyle are still there.